Sanitize phone numbers before storing them

This way, if people put extra characters (like -,.) we just
strip them out. Cool. So I came up with a regex pattern and then
made everything consistent across all the PHP. It might be
a good idea to also migrate the same pattern to the JS too.

Also fixes some template things. The subscribeSuccess template
had an extra closing div tag, so it was messing things up.

OH! This also puts a list of the subscribers on the admin page.