OAuth2 account linking / Integration

[vcfxb]

• Connect to Github
• Connect to Mattermost
• Connect to Discord
• Connect to RPI CAS

[vcfxb]

For RPI CAS, casbin looks like a good candidate: https://casbin.org/en/

[vcfxb]

GitHub & Discord linking in the backend should be done as of 41b6e0d

[vcfxb]

As of #181 Discord Linking is done. I'll close this issue and move GitHub linking to its own, since it's lower priority at this point.