This commit refactors `Rails::Command::Base` to more closely integrate
with Thor's help features, and to improve the base usage banner format.
With these improvements, subclasses can generally rely on Thor to
display nicely-formatted help.

__Before (example)__

  ```console
  $ bin/rails db:system:change --help
  Usage:
    bin/rails change [options]

  Options:
    [--to=TO]  # The database system to switch to.

  Change `config/database.yml` and your database gem to the target database
  ```

__After (example)__

  ```console
  $ bin/rails db:system:change --help
  Usage:
    bin/rails db:system:change

  Options:
    [--to=TO]  # The database system to switch to.

  Change `config/database.yml` and your database gem to the target database
  ```

__Diff__

  ```diff
  @@ -1,6 +1,6 @@
   $ bin/rails db:system:change --help
   Usage:
  -  bin/rails change [options]
  +  bin/rails db:system:change

   Options:
     [--to=TO]  # The database system to switch to.
  ```