Also consolidate Tesla mocks into the HttpRequestMock module.
Tests were not exercising the real codepaths. The Rich Media Preview only works with https, but most of these tests were only mocking http.
The Rich Media Previews were not regenerated when a post was updated due to a cache invalidation issue. They are now cached by the activity id so they can be evicted with the other activity cache objects in the :scrubber_cache.