Avoid multi-line ?: (the ternary operator). Use if/unless instead.

See #17478

This enables following cops:

Check for useless access modifiers

  Lint/UselessAccessModifier

Checks for attempts to use `private` or `protected` to set the
visibility of a class method, which does not work.

  Lint/IneffectiveAccessModifier

This also disables two false possitives in concerns.

https://github.com/bbatsov/rubocop/blob/v0.41.2/CHANGELOG.md

There are currently two cops for this:

* Migration/AddIndex: checks if indexes are added concurrently
* Migration/ColumnWithDefault: checks if columns with default values are
  added in a concurrent manner

Both cops are fairly simple and make no attempt at correcting the code
as this is quite hard to do (e.g. modifications may need to be applied
in various places in the same file). They however should provide enough
to catch people ignoring the comments in generated migrations telling
them to use add_concurrent_index or add_column_with_default.

This reverts commit 13e37a3e.

See #17478

Migrations shouldn't fail RuboCop checks - especially lint checks, such
as the nested method check. To avoid changing code in existing
migrations, add the magic comment to the top of each of them to skip
that file.

This reverts commit 3e991230.

See #17478

# Conflicts:
#	app/models/project.rb

See #17478

Do not use for, unless you know exactly why.

See #17478

See #17478

See #17478

No spaces before commas.

See #17478

See #17478

See #17478

Use Array.new with a block instead of `.times.collect` / `.times.map`.

See #17478

Favor `unless` over `if` for negative conditions
(or control flow ||).

See #17478

Checks of literals used in conditions.

See #17478

Do not use then for multi-line if.

See #7478.

Use def with parentheses when there are parameters.

See #17478

See #17478

See #17478

Use Unix-style line endings. See #17478.

See #17478

[ci skip]

Closes #17406

Also removes the note from the development/testing.md guide

[ci skip]