Case sensitive usernames

[brollb]

Although case-insensitive usernames are probably the better option (#30), there is a bit more work to make them happen (switch to user IDs, renaming user support). For now, we should use case-sensitive usernames to preserve backwards compatibility and make migration easier.

[brollb]

The main problem is that when migrating from the old system, we will get usernames with case differences. The current server converts everything to lowercase to make it case-insensitive but this will make accounts with capital letters unable to be logged into until a proper case-insensitive approach is taken (#31). However, that depends on a few other changes to make the migration from the old system work (mentioned above).