Make bot suitable for multiple servers

[TrojanerHD]

• [x] Change to database for storing guild-related settings

• [x] Make bot settings customizable via commands

• [x] Fix new bugs due to implemented features

  • [x] Wrong behavior / missing functionality when joining a new server

• [x] Make sure the command permissions can be updated (send authorize URL to executor of /permit command if required)

• [ ] Test if everything works as intended

• Closes #27

• [x] Will have to close #56 because the URL sending needs to be detached from starting the express server

[TrojanerHD]

Currently known bug: The bot has to be fully restarted in order to use the live and roles channel in a server it has been added during runtime

[TrojanerHD]

Edge case: If a user types /permit add <user>, then ignores the info message that authentication is needed and then /permit add <user> is executed again (either by the user themselves or by another user), the commands do not get reloaded (as in: the changes are not applied immediately)

[TrojanerHD]

Refresh tokens sometimes do not get stored and the whole permission system doesn't seem to work as the restricted commands don't get any custom roles that are permitted to execute them