Add user flags feature, template bug fixes and top rated template

Add a new "Top Rated" link in home menu to view the top rated songs, albums, artists.
Fix rating on stats pages and cell alignment when rating disabled.
Add a new 'user flags' to flag songs, albums and artists per user. This was a missing feature ; administrative flags doesn't have this purposen, and flag != rating. Feature disabled by default and has to be activated in config file. Once activated, it add a new Flag icon column near rating stars.

PS: I'm creating a pull request between two of my own branches because of pull request dependencies. I never did that with Github, not sure it's the way to do.