Update Link and Document sources to prevent confusing editing of entities without text selected

See https://github.com/springload/draftail/issues/85. Until there is a new API there, those two sources need to either:

Bail out of the entity adding workflow if the selection is collapsed.
Insert the text the entity is to be applied upon, and the entity.

I like option 2 better because it's closer to the behavior of other editors, and the text can always be edited / reverted afterwards. For links, we could go with the the page title for pages, and full URL (external or email links). For documents, the document title seems like a good link text.

springload / wagtaildraftail

Update Link and Document sources to prevent confusing editing of entities without text selected #57