Upgrade to Wagtail 2.13

[vsalvino]

[thenewguy]

Any timeline for this release? Running into this issue with embeds wagtail/wagtail#1745 which is resolved in 2.12 per https://github.com/wagtail/wagtail/blob/0daae4a2b829445ee97906dd4393398778881fcb/CHANGELOG.txt#L67

[FlipperPA]

@vsalvino Any chance we want to skip directly to 2.13 for 0.21?

[vsalvino]

A 2.13 branch is already in progress, based on this branch. If it needs a lot of work we may merge 2.12 in first, if it is a simple update then we will likely skip to 2.13.

The challenge with us is always the streamfield CSS, due to how we nest a lot of blocks. I need to hunker down one day and try and upstream our CSS!

[vsalvino]

See work in PR #418

[vsalvino]

Merged into dev