Make Seam context restoration (on select2) conditional via a widget property

Currently, we systematically restore seam context when calling an operation from nuxeo-select2.js as long as the 'currentConversationId' variable is available in a js closure.

So far, this is only needed for document suggestion to resolve additional page provider paramater (which could be el).

As a result, user and directory suggestion also restore the seam context uselessly which can lead to lock timeout issue when the user is typing fast in the select2 input.