Rethink data size limits

[PTKay]

Currently, the repository prefix size and the username size is too long for all repository options, as in:

• Max prefix size is 64 chars
• Max username size is 64 chars
• Repos are prefixed by "CG" and a number
• GitHub has a 100 char limit for repository names

This will cause problems when using long assignment names together with long usernames. The sizes of these names should be changed in order to support all possible repository names.