Remove mkt/ stuff from Olympia, second pass (bug 973150)

https://bugzilla.mozilla.org/show_bug.cgi?id=973150

This completely removes `mkt/` except `constants/` which are still referenced at various places in the code and `monolith/` because I was not sure of the implications.

Remaining failures:
* `market.tests.test_models.TestContribution`, `market.tests.test_models.TestPrice`, `market.tests.test_models.TestPriceCurrencyChanges`, `market.tests.test_models.TestRefundManager`, `market.tests.test_tasks.TestCheckPaypal`, `paypal.tests.test_views.TestDecorators` : I don't know if this code really applies to AMO. If it does, then that's something we need to fix but I'm not sure where to start. If it doesn't we have some more code to remove :)
* `stats.tests.test_cron.TestMonolithStats` : also unsure whether it applies to AMO or not

Some interesting greps for https://bugzilla.mozilla.org/show_bug.cgi?id=974850 :
```
% fgrep webapp lib/ apps/ -r | wc -l
289

% fgrep WEBAPP lib/ apps/ -r | wc -l
141
```