Isolate embeddable lookup by embeddable type in process_external_acti…

In LARA we are not sending very unique ID's to the portal (as we should)

Instead we are just sending the type and the id of the obj. in Lara.

Obviously the long range solution would be to make better ID's

But for activities that are already published, scoping to embeddable type seems like a good fallback.
(cherry picked from commit 5025c8769ab7a1833bfd43a429a803794e480cfa)
If the URL is blank, don't attempt to download its contents.

Should fix: [#59413108]
https://www.pivotaltracker.com/story/show/59413108
(cherry picked from commit 0d184809ad4e3bda602801bba901625873f8cd0d)