log open311 errors to database
better construction of open311 links in footer
add max_errors_logged config setting to log open311 recent errors

Makes logging conditional (because if max_errors_logged is zero, errors
won't be saved in the database. The table purges surplus records (which
normally should be a single record) after adding a new one; this isn't
very efficient, but it's really for setup/testing not prouction anyway.