Improve MR feature specs and reduce duplication

What does this MR do?

In this MR:

• Better organization of files:
  • spec/features/merge_request/ => single MR scenari
  • spec/features/merge_requests/ => MRs list scenari
• Better file naming: <actor>_<action>_spec.rb
• Better feature title, e.g. "Merge request > User sees merge widget"
• Usage of RSpec's default DSL: describe, let, before, it
• Removal of wrong-level assertions, e.g. expect_tokens
• Grouping of multiple it
• Extraction of shared behaviors to 2 new shared examples files

Next steps:

• Migrate features/project/merge_requests.feature to RSpec
  • Take the opportunity to remove duplicate scenari
• Further streamline redundant specs in spec/features/merge_request/ and spec/features/merge_requests/
• Apply the same rules and refactoring to other feature specs
• Use spec/features/merge_request/ and spec/features/merge_requests/ as examples of good practices