Unregister channel/group

Right now there's no way to unregister a channel/group.

The registration flow is completely handled by the `try_register` call in the `registration_flow` (https://github.com/galeone/raf/blob/main/src/telegram/handlers.rs#L1041) when a message is forwarded from a channel/group.

There's no way to unregister a channel/group.

Implementing this feature requires to:

1. Add a new command `/unregister`
2. Taking care of the active contests - forbids the deletion if there are active contests
3. If there are no active contests - then the deletion should work like there "ON DELETE CASCADE" feature of SQL. Evey record containing a foreign key pointing to the group/channel should be deleted.

There are 2 options for implementing point 3.

1. (discouraged) The re-creation of the table with the `ON DELETE CASCADE` statement. Discouraged since SQLite does not allow to alter the tables easily (it works only by drop table and recreation with the new schema).
2. (encouraged). Manually delete every dependant row (contests, ...), before deleting the main row in the `channels` table.

The schema structure is here: https://github.com/galeone/raf/blob/main/src/persistence/db.rs#L27