Additionally:
* It changes `purge` task on `sqlite3` adapter to recreate database file, to
be consistent with other adapters.
* Adds `purge` step when loading from `schema.rb`

When someone types in a generator command it currently outputs all generators. Instead we can attempt to find a subtle mis-spelling by running all generator names through a levenshtein_distance algorithm provided by rubygems. 

So now a failure looks like this:

```ruby
$ rails generate migratioooons
Could not find generator 'migratioooons'. Maybe you meant 'migration' or 'integration_test' or 'generator'
Run `rails generate --help` for more options.
```

If the suggestions are bad we leave the user with the hint to run `rails generate --help` to see all commands.

This PR fixes #8930 and some stuff from #8985

Let's say we just ran:

```
$ rails g migrate add_click_to_issue_assignment
```

We will get an error that looks like:

```
Could not find generator migrate.
```

This patch adds all existing migrations to the output to make it easier for a developer to find a valid migration.

```
Could not find generator "migrate". Please select a valid generator:
Rails:
  assets
  controller
  generator
  helper
  integration_test
  mailer
  migration
  model
  resource
  scaffold
  scaffold_controller
  task
```


It would be nice to do some spelling detection and suggest alternatives, but for now this should  help.

Minitest has already been required when calling Minitest.autorun.

The fix introduced at 75ba7425 were
missing this case.

To fix the original issue mentioned at that commit the correct is to
remove update_config_files from the command list since we should not run
both config_files command when creating a new application.

Tests were added to make sure everything is working fine.

Since the generated application’s Gemfile is using single quotation marks, it
seems fair to use it on plugin’s Gemfile as well.

Database configuration was trying to load the first path that
config.paths['config/database'] was returning even if the path didn't
exist in the filesystem.

Because Rails::Paths::Path has the possibility to return more than 1
path (as an array), database_configuration should filter down the paths
to the existing one and then load the first one.

This would make it possible to move the database.yml file and add the
new path to paths['config/database'] and still load the configurations.

Let's avoid useless output when testing stuff related to Bundler.

You can't compile a file that references a non-compiled asset's path.

.. unless you turn off asset runtime errors.

Outside of a test case like this, `config.assets.precompile` would
normally retain its default entry, which precompiles all images (and
other non-JS/CSS files) that are in `app/assets`.

Like rake tasks and runner blocks these blocks should also being shared
between applications since they are stored at the classes.

Fixes #14748

sprockets-rails 2.1 needs the precompile list to be available in all
environments.

Fixes #14620

Restore the 4.0 behaviour for 'sqlite3:///', but deprecate it. We'll
change to the absolute-path interpretation in 4.2.

The current "correct" spellings for in-memory, relative, and absolute
URLs, respectively, are:

    sqlite3::memory:
    sqlite3:relative/path
    sqlite3:/full/path

Substantially reverses/defers fbb79b51.
Uncovered by @guilleiguaran while investigating #14495, though that
sounds like a different issue.

  ActiveRecord::Base.pluralize_table_names = false.

  Previously, generation a migration like this:

      rails g migration add_column_name_to_user name

  would not generating the correct table name.

Fixes #13426.

If the sqlite file name change in future version we this regexp should
catch

This is a test case for the fix provided in eafec469

Just pass in the expected database as argument rather than "caching"
it in a hash and using the hash in the helper methods.