Add tag search when assigning tags

[jasonaowen]

On the new / edit project pages, the first iteration on tags (#62) lists out all the tags in the system and asks users to check them off. The list of tags is only going to grow, so instead of listing all of them, use JavaScript (depends on #70) to keep the list hidden, present a free-form text field, and match what the user types to the list of unselected tags.