Select skin test group and persist selection (#1267)

Select skin test group and persist selection (#1267)

Select skin test group and persist selection

* Rename "Groups" to "Buckets" in campaign definitions
* Add Bucket class instead of using strings
* Add code for sorting visitors into buckets, depending on URL parameters and cookies
* Persist bucket selection cookie
* Replace skin changing code with skin changing campaign (only active on dev)
* Remove old code for A/B testing payment methods

Features: 
* https://phabricator.wikimedia.org/T196335
* https://phabricator.wikimedia.org/T192635

Merge pull request #1264 from wmde/migration_support

Merge pull request #1264 from wmde/migration_support

Add support for migration scripts

Add support for migration scripts

Add support for migration scripts

T196333

Merge pull request #86 from wmde/update-eslint

Merge pull request #86 from wmde/update-eslint

Update eslint to be compatible with Wikibase .eslintrc

Update eslint to be compatible with Wikibase .eslintrc

Update eslint to be compatible with Wikibase .eslintrc

Merge pull request #1272 from wmde/add-styling-for-links

Merge pull request #1272 from wmde/add-styling-for-links

Add links styling to all content pages

Add links styling to all content pages

Add links styling to all content pages

Bug: T196755

Merge pull request #150 from wmde/address_change_entity_constructor

Merge pull request #150 from wmde/address_change_entity_constructor

Fix potential DB pagination issue

Fix potential DB pagination issue + formatting fix

Fix potential DB pagination issue + formatting fix

T196333

Merge pull request #1266 from wmde/fix-cache-cleaning

Merge pull request #1266 from wmde/fix-cache-cleaning

Clean cache in docker containers