Improve visibility of permission requirements

[Intery]

Idea

It is often currently unclear what permissions the bot requires while setting up, and, in operation, whether the bot has the permissions it needs.
These need to be made more explicit, particularly during the initial setup process.

Implementation

• Add an admin-locked checkpermissions command which checks whether the bot has the required permissions for each study group
• Add a permissions dialogue after or during the newgroup setup process which checks whether the bot has the correct permissions to manage the study group being set up.
• Add constructive hints to the permission failure messages such as "I don't have permissions to add the study group role"

[Intery]

Partially implemented in 9634af1