Fix #1169

https://github.com/mybb/mybb/issues/1169

1. Hides empty profile fields.
2. Hides empty website completely.
3. Hides contact details if empty.
4. Hides the PM button if it can't be used by the viewing person.
5. Hides the signature if the user has no permission to use it.
6. Adds missing <br /> in away status.
7. Fixes alt_trow() so it always alternates the background.

Note: not tested thorougly yet.