Don't prematurely translate topics#show

Topics aren't translatable, so no need to complicate the translation
files just yet.
Don't unnecessarily include routing helpers
Tidy up topics functional/acceptance tests

I've made an attempt to pair down and simplify the testing around the
management of topics as there was (IMHO) excessive and brittle
functional testing here. We are still to have a wider team discussion
around testing and the approach we take, but my focus here has been to
end up with a core set of functional tests that give us broad
confidence that are controller code is doing what we expect without
excessively testing every last detail. To really tidy things up, we'd
ultimately have to move much of the logic out of the controller itself
(perhaps moving things to a separately testable presenter), but for
now, I've done the following:

* Reduced the specifity of selectors when testing content on views to
make them less brittle
* Avoid testing behaviour that is covered adequately elsewhere
(e.g. cache control behaviour, scopes loading correct things, etc.)
* Avoid testing behaviour that is unlikely to break.
* Stick as much as possible to the happy path, especially in cucumber
scenarios