This adds experimental support for a microversion selector. The
interface for this may well change over time. To currently use this we
need 2 values in the sphinx conf.py to specify min and max
microversions. If you don't have those added, nothing is changed in
the UI.
Use at your own risk. However it's much easier to get feedback if we
have this in the package.
Change-Id: Iaf7e117ea5792bb3b60c1fd0ae416b241ab89d65
f93c467e9a