Remove association between posts and attachments

It wasn't really being used anymore, but old code was left there.

refs #454
Restrict the scope of action of the js in invitation to webconfs

The js should only be applied for the content inside the modal used to send
invitations to web conferences. But it was not being scoped to this modal,
so it was being executed every time any modal was opened, resulting in js
errors sometimes.
Form to add posts to spaces now in a modal, not in the sidebar

All other forms to add resources are usually either in a new page or in a
modal window. The form to add posts was the only one that deviated from this.
Removed it from the sidebar and moved to a modal window.
Simplify and clean the view posts/index

Show all posts in the same list, not separated by date anymore.
Also cleaned it up a bit, removing unnecessary code.