Don't try to return boundary changes for places with no MapIt area

We were getting error emails for a couple of places that had no MapIt
area associated with them - these are historic data where it's going to
be difficult to get missing data, so let's just settle for not showing
boundary changes for these cases.

Fixes #1335 and #1377.
Merge branch 'fix-for-missing-mapit-areas'
Add ability to view all Hansard appearances for a person.
Merge branch 'mit-pages-squashed'
MIT: Add a link to return after senate / petition submission
MIT: Fix county performance view tests

Since 62b96fbd9858 we only record a page view if utm_expid is in the
query parameters.
MIT: keep experiment data in the session rather than query parameters

Generating a user key on each view of the county performance page, and
passing it on as a parameter, means that if the same user returns to
the page later (e.g. by pressing "Back" after a sentate comment
submission) they'll probably have a different user key - this is
problematic, and we can easily get around it by storing the user key,
variant and facebook demographics in the session.
MIT: Add some docstrings and comments