User should not be able to mark recurring channel reminders as completed

[kemenaran]

Summary

Currently it is possible to mark a recurring channel reminder as "Completed". This doesn't really make sense, as normal (i.e. personal) recurring reminders can't ever be marked as complete.

This means the reminder will continue to trigger recurrently, without being visible in the reminders list.

Steps to reproduce

Steps to reproduce the behavior:

1. Add a channel recurring reminder: /remind ~general "Daily meeting!" every weekday at 09:00AM
2. Display the reminders list: /remind list

Expected behavior

The reminder displayed in the list can't be marked as "Completed".

Observed behavior (that appears unintentional)

The reminder displayed in the list has a button to mark it as "Completed".

[Roy-Orbison]

Related to #174 where I proposed cloning the reminder as single event when it occurs. This would probably solve this issue as the clone rather than its parent recurring reminder could be marked completed.