Now only "<numbers>/<numbers>" can exist in the ba_page field

Was tested with a few sample ba_page's

I made this pull request so that when checking for a duplicate beer the site has a better time finding one if it exists.

Will likely need to run the following UPDATE MySQL query to get everything back in line (I've already ran this, so this is just in case someone else adds a beer between now and when you get to this pull request):
`UPDATE beers SET ba_page=SUBSTRING_INDEX(ba_page,"/",2) WHERE ba_page REGEXP '^[0-9]+/[0-9]+/.*$'`