feat: Switch to Merge Requests API to fetch assigned MRs

Iterating over all projects the bot is a member of, then fetching
assigned merge requests for each can take a considerable amount of time
when the bot is a member of many projects, which is easily achieved when
adding marge-bot to a namespace. Make use of the Merge Requests API to
shortcut this process, and directly fetch all merge requests assigned to
the bot.

Closes #97.