Update pinned and linked fields

The bot should automatically update the values of the pinned and/or linked fields when an issue link is added, updated or removed.

@calebsander said:
Do wrote a link not a long time ago, and it totally spammed the activity feed for days

While spam shouldn't be a huge problem (remember the flood when Arisa started removing redundant labels? That lasted for more than a week!), I guess you could add this feature as an @updtd_module so that it's only triggered on issues that are updated by someone.