Follow up to #23508.

`schema_type` returns symbol rather than string

Added numeric helper into `SchemaStatements` for MySQL and PostgreSQL

doc changes to clarify asset pipeline

for rails/rails#23431

       modified:   guides/source/asset_pipeline.md
* description of asset combination from apps and gems, e.g. jquery-rails
* after @vipulnsward's related change rails/rails#23479
  correction: --skip-sprockets will prevent all of these gems, not just sass-rails and uglifier

       modified:   guides/source/working_with_javascript_in_rails.md
* noted that rails.js requires the asset pipeline

[ci skip]

Hand off the interlock to the new thread in AC::Live

A return value of `schema_type` is used by:

1. primary key type: using as `symbol.inspect`
2. normal column type: using as `symbol.to_s`

It is better to return symbol.

A full write-preferring wait can lead to deadlock.

Publish guide on "Using Rails for API-only Applications"

Fixes #23322
[ci skip]

[ci skip]

With this addition, you can add a column into the table like:

```
  create_table(:numeric_types) do |t|
    t.numeric :foo, precision: 10, scale: 2, default: 2.0
  end
```

The result of the migration above is same with:

```
  create_table(:numeric_types) do |t|
    t.decimal :foo, precision: 10, scale: 2, default: 2.0
  end
```

fix typo in `assert_enqueued_jobs` example [ci skip]

Most importantly, the original request thread must yield its share lock
while waiting for the live thread to commit -- otherwise a request's
base and live threads can deadlock against each other.

MariaDB does not support JSON type

Referencing Rails.env without checking if it's defined couples
us to Railties.

Fix by avoiding the line breaks if we don't have an env check
to rely on.

Remove unused private method

Remove duplicated `require 'arel'`

Fixes #22980.

Replace old Rails greeting references

It appears first in `lib/active_record.rb`.

Revert "No need to specify Ruby patch version on Travis CI"

Revert "When generating a mailer, you must specify Mailer in the class name in"

A couple of the READMEs were still referring the old welcome page. This
is a small change that goes over it.

[ci skip]

This reverts commit 8417d967.

In 5697bdbb and af3eb596,
add mailer suffix to generated files and classes.
Therefore, no longer need to specify `Mailer` to class name. [ci skip]

Follow up of https://github.com/rails/rails/commit/c9feea6c9ab4494b0cb0b8cf4316847854f65af6

Previous commit accidentally broke mixing line filters with string -n filter.

Fix by checking if it is a string and returning it.

We also need to ensure the -n filter carry forward into any other composite filters.

Fix by letting the named filter be extractable, so we'll keep this for the next runnable's
run.

This reverts commit 99801c6a.

Ultimately it doesn't matter whether `add_index` or `t.index` are used
in the schema dumper in any meaningful way. There are gems out there
which hook into the old behavior for things like indexing materialized
views. Since the reverted commit doesn't seem to add much benefit,
there's no reason for us to break these gems.

No need to specify Ruby patch version on Travis CI