Various fixes and improvements related to layout and UX

Issue https://github.com/AUTUpStageTeam/UpStage2013/issues/131 (Pressing enter will submit changes or invoke an appropriate action for whatever the user is doing)
- Close alert box (on every page)
- Submit new user's email/password
- Create new user (when the focus is either on the second password text box or email text box)
- Submit changes on media edit page (Edit, Assign, Tag)
- Search on media page

UX
- Stage edit page now uses alert box (with shaded background to show media preview) and dismissible with the enter key (related to https://github.com/AUTUpStageTeam/UpStage2013/issues/163)
- Editing other users will now redirect back to editplayer page
- Selecting a stage from the dropdown box will immediately update the stage page
- Selecting filters will update the media table immediately

Styling
- Center the alert box
- Stage edit page overall layout
- Stage edit page button area collapses to the left on small screen
- Media upload page https://github.com/AUTUpStageTeam/UpStage2013/issues/162

Code
- Moving away from using form numbers to build request strings
- Reusable alert box