Upgrade rubocop and rubocop-rspec

Context

Closes #36 (closed)

Rubocop and rubocop-rspec are outdated, and they prevent us using new Rubocop cops (e.g. gitlab-org/gitlab#356738 (comment 952865877) or #34 (closed) or #35 (closed)).

We should follow the upgrade guides when updating both those gems:

• Rubocop 1.x upgrade guide
• rubocop-rspec 2.x upgrade guide

What does this MR do?

• Upgrades Rubocop gem from 0.91.1 to 1.30.1
• Upgrades rubocop-rspec gem from 1.44.1 to 2.11.1
• Changes the codebase so that we have green tests (there were a lot of changes to be made)
• Added a playground folder with a rails app that has this gem installed. It also has some Rubocop offenses in order to test the gem locally.

How to test it locally?

• Method 1: Run the specs
• Method 2: Check the directions in the README about the playground folder.

Review suggestion

Go commit by commit:

• Commits prefixed with [RuboCop] are the result of strictly following sections of the Rubocop upgrade guide.
• Commits prefixed with [rubocop-rspec] are the result of strictly following sections of the rubocop-rspec upgrade guide.
• The rest of the commits make the test suite and the rubocop command green.