WL-3583 Add values m.ox needs to render announcement.

In the upgrade to Sakai 10 some values had got dropped from the announcements entity broker API. These meant there wasn’t enough information to construct an entity URL for an announcement and also m.ox failed to render as properties it expected weren’t present.

Without a getId() all the entity* values don’t get generated.
Merge pull request #7 from ox-it/WL-3583

WL-3583 Add values m.ox needs to render announcement.