Implements #542 https://tangled.org/did:plc:j5hmlfdrwkvtxm7cjmu7j2is/issues/542
Adds a "opened a pull request" events to the timeline.
When a user creates a PR, it will appear as a card showing the author, target repo, title, number, state, and a body preview.
I've also added a distinction between the global and following feed (userIsFollowing), for the future.
Changes were tested locally. Screenshot: https://returnnull.de/merge_request_timeline.png