Sanitize tag list and title when adding/editing a source

I was getting slightly annoyed with having to use trim() and strlen() all over the place, so this cleans up the tag list into a nice, trimmed list when inserting or updating a source in the database.

Snippets like these:

    foreach ($source['tags'] as $tag) {
        $tag = trim($tag);
         if (strlen($tag) > 0) {
             ...

would now be redundant, but I don't feel like trying to find them all.