Closes #17406

[ci skip]

Almost all cops are starting as disabled until we can fix their
violations.

This enables multiple Rubocop cops that already conform to our codebase
and do not require fixes.

Also fix one use of `gsub` that would be faster as `delete`.

Use `tr` instead of `gsub` when you are replacing the same number of
characters. Use `delete` instead of `gsub` when you are deleting
characters.

Also fixes the errors caused by enabling the cop. casecmp is more performant than `.downcase` and `==`.

This reports uses of `File.exists?` and `Dir.exists?`, which were both
deprecated in Ruby and will eventually be removed in favor of
`.exist?`. Also fixes all existing uses of the deprecated methods.

This requires no code changes since it doesn't actually change anything
in the codebase, just preventative.

To 60.

Discussed in #14233.

See [their releases](https://github.com/bbatsov/rubocop/releases) for
more info.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.

This enables rubocop metrics like CyclomaticComplexity and ABCSize.
Initial threshold values are high, should be probably decreased.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>