Add expires field to web UI

This adds the expires field to the web UI. Not much to say about it. I use the angular-ui bootstrap component DatePicker.

Had to mangle some faulty CSS which messed up the look of the datepicker.

I'm quite certain that timezones are not correctly handled. I'm not quite sure how we should do it so I'm deferring that for the future (tricky part is likely that client and web server is in different timezones, so we can't rely on the local timezone of the web server).

Fixes #599.
Hm, this is probably the same issue as I found yesterday. As the prefix form template is included with ngInclude, it gets its own scope instead of using the controller's scope as one would expect. I guess using $parent violates some rules, but let's go with that solution for now.