[WIP] case insensitive username

[nitely]

fixes #178

todo:

• data migration
• case-insensitive mentions
• make usernamelower on registrarion, login,
• use user.st.nickname on templates
• populate user.st.nickname on profile creation
• case-insensitivity on private forum invitations
• data migration tests
• case-insensitive mentions tests
• usernamelower on registrarion, login tests
• populate user.st.nickname on profile creation tests
• case-insensitivity on private forum invitations tests

[coveralls]

Coverage increased (+0.07%) to 97.176% when pulling b10a54b on usernames_ci into 1bd3608 on master.

[nitely]

This is a breaking change. Unless we make the setting off by default, but first I like case-insensitive (CI) usernames better and second the feature can be turned off without running into conflicts, that's not the case when turning it on.