The urls that we generate for mobile post notifications don't take into
account the subfolder url if a site happens to have one configured. When
this happens when you tap on a new mobile notification it takes you to
a url that doesn't work because it is missing the subfolder portion.
I honestly think this should be handled in the Post model like we do
with the Topic model. `Post.url` should know how to handle subfolder
installs, but that seemed like a very risky change because there are
lots of other places in the codebase where we tack on the base_path and
I didn't want to risk duplicating it.
I also found a small typo in the topics controller spec.
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