Merge Request API Endpoints to use iid instead of id

MR API uses the id of a MR. Would be useful to be able to use also the value of the iid.

For example, the Accept MR API would be easier to use: Currently to use the API to perform this task, you need to get the MR ids first then perform the action. Instead of

PUT https://gitlab.com/api/v3/projects/1331064/merge_requests/2/merge

You need to use:

GET https://gitlab.com/api/v3/projects/1331064/merge_requests
PUT https://gitlab.com/api/v3/projects/1331064/merge_requests/800515/merge

Either this or provide the DB id somewhere in the UI so its easier to handle