v0.1.6

This release fixes the URL written in go.mod. Additionally:

- The course number update delay is now dynamic based on the number of
  connections and the number of courses
- Previously, if a user tries to choose multiple courses in a group, the
  choice is added to the database, but then the groups are checked and
  the connection is closed and the integrity check fails in future
  connections. Now we reject it at the right moment.
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