Improve quota input field

- Added tooltip with examples
- The quota value returned by the server is now displayed after update
- Fixes #5346

Check this out @jancborchardt @kabum @raghunayyar @bantu @blizzz 

I haven't tested in IE8 and others yet. (only Chrome)

I think this should be enough for OC6.
Validation can come in OC7.

In the current situation, if you input a string it will transform itself to "0 B" which is ok, at least you get feedback about what the intepreted value is.