api: expose web_url for project entities

What does this MR do?

Adds a web_url field to project snippet, issues, and merge request API returned objects.

Are there points in the code the reviewer needs to double check?

Tests. Are they sufficient? (I'm working on getting them to pass.)

Why was this MR needed?

Creating the URL for an MR or issue from the MR is tedious in API libraries; it's easy for Gitlab to just provide it.

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together