Change the modal classes to use the lib bootstrap-modal

It's simpler than the lib that was being used (dialog2), doesn't assume as
much things (doesn't automatically run callbacks on the buttons in the modal
footer, for example) and uses bootstrap's syntax. Also seems more up-to-date.
So it's overall better than the one being removed.

Version added: jschr/bootstrap-modal@3731fd5f5f3
Fix a fix wrong links opening and closing webconfs, refs #804

Start and end links were not 100% correct for both user's and spaces' rooms.
Simplify the confirmation dialogs a lot and make it more standard

Now they use our modal classes, so it's just like showing any other modal
in the application. The code is also a lot simpler, with a proper template
to generate the html and with localized strings.