SVG image support (requires dependency chain updates)

[stevejalim]

Wagtail 5.0+ now supports SVG images in the core Images section, which is something we're very likely to need, particularly in hero headers. SVGs are better than bitmaps

We're currently on Wagtail 4.2.x LTS, but we can move up to 5.x because 5.2 LTS is due to land in November 2023.

However, to get to use Wagtail 5.0+, we need our dependencies to also support it. Wagtailstreamforms does NOT work for 5.0 yet

It seems wortwhile to contribute a fix/update to wagtailstreamforms, as a way to retain the form behaviour which will have significant value while unlocking SVG image support

[stevejalim]

Done in #55