Send a periodic summary of activity

As an optional feature, it would be quite cool for Hookshot to track and announce the changes that happened over a period in a given room.

E.g. if your room is connected to GitHub and GitLab then a summary of the last week's actions would be nice.

There are obvious questions about how this summary would be stored (perhaps account data, that rolls around weekly).

No files, tests, or entry points are identified in the issue. Start by tracing how GitHub and GitLab activity is currently handled for a room, then determine how weekly summaries and their account data would fit; done requires an agreed storage and scheduling design plus a defined summary format.

Written by the indexing model from the issue text.