Wagtail preparations

[nrennie]

For charts published on Florence but embedded in Wagtail publications there are a few differences:

• No source text from charts as now a Wagtail input
• non-JS fallback image required
• iframe title in pym (maybe)

Updates to main can be merged into wagtail-dev (@nrennie responsible) but wagtail-dev should remain separate from main until no more embeds on Florence publications. In branch #481.