Rewriting the CN UI (Part 1)

In this installment I introduce the new banner editor
based on HTMLForms. This patch relies on the following
core 1.22 patches to HTMLForm:

https://gerrit.wikimedia.org/r/#/c/55009/8
https://gerrit.wikimedia.org/r/#/c/59063/4
https://gerrit.wikimedia.org/r/#/c/59439/

Still to come in later installments:
- Inline frame previews of banners
- Banner archival
- Selection of banner mixins
- Campaign editor HTMLFormization

Change-Id: I02b1f7f5f5dff7e95528b7c55105f7447227b329
Merge "Rewriting the CN UI (Part 1)"