Wrote class for single proposers

Similar to the other classes: we have one class handling a single proposer and
another handling a list of them.
Implemented tag page

Most tags are now links, except where they shouldn't. These links lead
to a page listing all problems having that tag. Further, a top 5 list of
proposers of problems with that tag is presented.

Along the way, we also cleaned h3's up: they don't need shadows if we
just make them bold.
Improve global body layout
Created proposer page

Show all problems for a specific proposer. A tag statistic is displayed
as well. The page still lacks a list of solutions.
Link issues in task lists
Deny access to .ht* and .git* files in Apache
Added nginx to README, rewrapped to 80 chars

Also clarified statements about rights a bit.