Previous to this change an optimisation stripped crawler content from
all mobile browsers.
This had a side effect that meant that when we dropped support for an old
mobile platform we would stop rendering topic and topic list pages.
The new implementation ensures we only perform the optimisation on modern
mobile browsers.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
