Merge pull request #590 from ednapiranha/master

added BASKET_URL in settings_base.py (bug 839229)
start a purchase (bug 839253)
hide themes stuff behind waffle on add-ons devhub dashboard
do not validate empty search fields (bug 839049)
fix test
Fix for @cvan's payverlay regression (bug 838186)

Removed potato-initialization of form submission handler.
Bump amo-validator

+ Added compat tests for FX19
* Cleanup of compat tests
Bump amo-validator

- Spurious debug statement
Fix for modaled filters dialog

The header is no longer underneath the modal.
unleash payments in USA
stop commit error when running celery as eager