added favicon.ico to avoid serving 404s on every request
change registration email around to avoid getting marked as spam
email users easier from list_users page
nicer messages when requesting a non-existent user/challenge
better is_valid_id function
auto-focus on two factor auth form
don't show user browser in exception list
use array_get instead of checking isset and then getting
fix category checking logic on challenges page
better pagination
absolute links in list_users
better file ID validation
remove user_agent_full, add unread flag to exceptions
Tooltips working correctly

Uses tabs
Merge branch 'master' into challenges-page-tweaks