Create `simple` mode for merge requests API
As seen in https://gitlab.com/gitlab-org/gitlab-ce/issues/34159#note_34634821, the merge requests API takes a bit of time because it has to load notes, merge request diffs, etc. into ActiveRecord. The Trello integration just needs basic information:
- Name
- ID
- Link
It appears the project API already has a basic endpoint that uses the simple flag: https://docs.gitlab.com/ee/api/projects.html#list-a-user-39-s-projects
We should just do that, and we can be sure that it will be fast and can easily be done.
/cc: @smcgivern, @jneen