Fix UI Themes pagination, tar.gz/.bz2 support

Closes #72
Closes #86

Pagination was cutting links off after whitespace (lack of %20 text
replacement).  Fixed with php `rawurlencode` function.

tar.gz and tar.bz2 files couldn't be uploaded or edited due to the way
file extensions were being parsed.  Added dedicated function
`parse_extension()` to utils.php which makes special consideration for
files with .tar. in the end of their name.

Also added text-center div to center pagination links.