Set up `RuboCop` based on `Standard`

[chubchenko]

Set up RuboCop based on the Standard rules + fix all RuboCop issues.

• Create a separate rubocop.gemfile and add it to Gemfile

• Add rubocop job to CircleCI

• Correct all layout issues

• Use double-quoted instead of single

• Avoid comma after the last item of a hash and an array

• Use if instead of empty case (Do not use empty case condition, instead use an if expression)

• Use $stdout instead of STDOUT

• Use safe navigation (&.) instead of checking if an object exists before calling the method

• Add frozen string literal comment

• Use other as a name instead of collection/factory/trait

• Remove unnecessary symbol conversion

• Use caller_locations(2..2).first instead of caller_locations[1]

• Move class methods inside a class << self block

• Check block argument explicitly instead of using block_given?

• Use eval_gemfile instead of eval

[djezzzl]

Please let me know when the PR will be ready 👍

[chubchenko]

@djezzzl It's ready for review 🎉

[djezzzl]

Thank you a lot for the contribution!

Could you please say if you have any plans to look at database_validations and database_consistency gems? I'm asking to know if I should apply such great improvements by myself or I can count on you 👍