Add at option to perform_enqueued_jobs test helper

Currently, the `perform_enqueued_jobs` helpers will also immediately
perform jobs that are scheduled via `set(wait:)` to run in the future.
This commit adds a new argument to `perform_enqueued_jobs` to make it
only run jobs scheduled at or before the passed in `Time`. This allows
testing the side effects of immediate job execution separate of jobs
delayed in the future.

[ci skip] Mention `context` option of save in documentation

`ActiveRecord::Validations#save` takes `context` option
which can change validation context.
This fact should be mentioned in the documentation.

Update rails-html-sanitizer to 1.1.0 in actionview.gemspec

Commit https://github.com/rails/rails/commit/52f0b050e25cac6d9571d71c9f74ea583d8aa2b0 replaces `white_list_sanitizer` with `safe_list_sanitizer`. This is a breaking change unless the installed version of `rails-html-sanitizer` is `>= 1.1.0`.

This commit updates the minimum version in `actionview/actionview.gemspec` to `1.1.0`.

Added multiple database Active Record guide entry to the release notes

Corrected config option path in release notes for deprecation release notes of Rails 6 [ci skip]

Corrected deperecation config option path for active_record.sqlite3.represent_boolean_as_integer option in release notes. [ci skip]

Fix cookie modification during rotation

- The sanitizer has been changed to safe_list_sanitizer.
- deprecate white_list_sanitizer

Added the ability to initialize `thread_mattr_*` methods with default
values like so:

``` ruby
  class MyClass
    thread_attr_reader :foo, default: :foo
    thread_attr_writer :bar, default: :bar
    thread_attr_accessor: baz do
      "baz"
    end
  end
```

This is consistent with the api exposed by `mattr_accessor`.

Fix `PG::ConnectionBad` error when running fixtures

Use media_type instead of content_type internally

These calls to `content_type` were triggering the deprecation from
c631e8d0 in upgraded applications.

We can use `media_type` in all of these cases to avoid the deprecation.

Didn't like the complicated stuff that happened on credentials:edit. It
would append to .gitattributes multiple times. Though I see why it was
written that way.

I'm cutting off for now, but since this new flow would require each developer
to run --enable perhaps this should really be:

1. Developer enrolls Rails app by running `credentials:diff --enable`
2. credentials:edit checks .gitattributes for `diff=rails_credentials` and
   if the current file is covered by that.
3. If so, set up the "rails_credentials" driver automatically.

Helpers is more for sharing between commands. Since `Diffing` is only
for credentials we should just keep it only for credentials.

Replaces "pretty" with diffing since the former is ambiguous, while
diffing captures what it does. `opt_in` seemed clunky so it's swapped
for the one-word enable.

Address `DEPRECATED: Use assert_nil if expecting nil`

```ruby
$ cd activerecord
$ bin/test test/cases/dirty_test.rb:494
... snip ...
DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/git/rails/activerecord/test/cases/dirty_test.rb:494. This will fail in Minitest 6.
DEPRECATED: Use assert_nil if expecting nil from /home/yahonda/git/rails/activerecord/test/cases/dirty_test.rb:511. This will fail in Minitest 6.
.

Finished in 0.061593s, 16.2356 runs/s, 795.5428 assertions/s.
1 runs, 49 assertions, 0 failures, 0 errors, 0 skips
$
```

Refer seattlerb/minitest#666 rails/rails#27712

typo in word compatibily -> compatibility [ci skip]

At first this appeared to be a multi-db bug but after some invesitgation
it was clear that this can occur just by calling `establish_connection`
from ApplicationRecord.

After some investigation we found that this only occurred when using
fixtures. The console boots fine, the server runs fine, and the tests
even run fine if we used paralellization or eager loading in the tests.

I tracked the issue down to the line that calls
`self.connection_specification_name = name` in the SchemaMigration
changes for Rails 6.0. But how can this be? That is not that major of a
change? How could `connection_specification_name` be a problem?

First `connection_specification_name` caches the name of the connection
specificatio. Second, fixtures were incorrectly holding onto a reference
to that connection.

So when you went to run the tests the models wouldn't be connected and
when the fixtures tried to load the data it would choke on that
unconnected database.

The changes here move the connection into a lambda so we can call it
when we need it rather than blowing up before the model is connected.

Fixes #36743
Co-authored-by: NAaron Patterson <aaron.patterson@gmail.com>

Usually the application requires the entire active support at load time
but the configuration happens before it is loaded. For that reason we
need to require the core_ext that we want to use in this file.

Add ability to unset preventing writes

Previously if an app attempts to do a write inside a read request it will be
impossilbe to switch back to writing to the primary. This PR adds an
argument to the `while_preventing_writes` so that we can make sure to
turn it off if we're doing a write on a primary.

Fixes #36830
Co-authored-by: NJohn Crepezzi <john.crepezzi@gmail.com>

Introduce InvalidConfigurationError

This reverts commit dd779c96.

This reverts commit 98d0f7eb.