WIP: Add API for merge request templates

What does this MR do?

Adds an API to the merge request template: GET projects/:id/merge_requests_template and POST projects/:id/merge_requests_template

What are the relevant issue numbers?

Resolves #885 (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