Remove the `Gitlab::Routing` module and use Rails' CustomUrlHelpers

Why are we doing this work

Since the Rails version 5.1.0, it is possible to define the custom URL helpers by using direct DSL helper which can replace using our own Gitlab::Routin module.

To make the process of defining the custom URL helpers, we should remove the Gitlab::Routing module and move all the custom URL definitions to the direct DSL method.

Relevant links

• This is a follow-up issue for !51865 (merged)

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• frontend Step 1
  • @person Step 1a
• frontend Step 2