This will make it easier to move the loading portion into a separate
gr-diff-host component while keeping the rest in gr-diff.
This will delay computing the weblinks until the image diffs are loaded
if there are any. I think that is very minor because creating these
links is very cheap, so it should not really matter.
`finally` is used for loading instead of `then` because that is more
robust in the case where any of the early steps throw an error
(currently they don't, but in future they might).
I had to change two tests a bit because they were testing implementation
details instead of the API - I think they are better now.
Bug: Issue 9623
Change-Id: Ic4f8706e902d333da622580e3231944bd24cdeb0
9e98b0b03e