Resolve "All Gitlab branches listed in Jira task in Development section"

What does this MR do?

Changes:

1. Jira OAUTH endpoint is now /login/oauth instead of /-/jira/login/oauth

This change was necessary because if you set up Jira with a path (our instructions suggest you use /-/jira) then Jira will use that path for all requests instead of using proper API endpoints

2. All github mimic layer API endpoints now accept :namespace/:project params

Please note that I did not keep the old endpoints because -/jira is picked up by :namespace/:project anyways. I added tests to confirm this.

These changes now permit Jira to ask for branches/MRs for ONLY the relevant project, fixing the duplication issue in #6752 (closed)

IMPORTANT This MR introduces a path: /login/oauth. In my testing adding this path doesn't conflict with anything else but it's important for any reviewer to check this (please!)

What are the relevant issue numbers?

#6752 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #6752 (closed)