Adds a MediaWiki cookie prefix parameter. This fixes a bug that prevents users from logging in to Scripto when using MediaWiki 1.27.0+. The fix should be backwards compatible, so if you have an existing Scripto installation that works, it should still work without any changes.
The cookie prefix is predictably in one of the following formats:
<dbname>if a table prefix is not used, -or-<dbname>_<tableprefix>_if a table prefix is used
<dbname> is the MediaWiki database name and <tableprefix> is the MediaWiki table prefix, in case one is being used. (Note the trailing underscore in the second format.)