Update the DDG tag line, remove link and make sure full stop is included.

I made a mistake when tokenising the tag line on the front page and included punctuation in a token, so some translations drop it, others include it.

This adds a small regex match function so we can look for the full stop and include it if it is missing.

Next time the tag line is updated, all of this can be removed, though I think the match function is potentially useful.

There are other approaches to this, including attempting to update the token in the database or re-publishing the tagline for translators (this is my least favourite). Thoughts?