Sanitize tag list and title when adding/editing a source

[seanrand]

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 ($tags as $tag) {
    $tag = trim($tag);
     if (strlen($tag) > 0) {
         ...

would become redundant. (Well, except for tags that have been added to the database before this change, until the source is edited again...)

[jtojnar]

Hi @seanrand. Previously, it was not clear if selfoss was licensed under GPL 3 only, or also any later version. Could you clarify whether you are fine with licensing your contributions here and in other PRs under GPL-3-or-later?

Thanks again and sorry for the confusion.