✨ add SVG support for logos

[ldericher]

This is my proposal to use an SVG as .Site.Params.Logo.

Additions, hints and corrections are welcome!

[ldericher]

I can see my PR will not handle .Site.Params.SecondaryLogo correctly as it is replaced using JS in

blowfish/assets/js/appearance.js

Line 80 in 18ed67b

elems = document.querySelectorAll("img.logo")

This might be irrelevant aka. "you should create a reactive SVG using CSS in that case"

[ldericher]

Appreciate the merge, but this was WIP and I'm gonna propose another change instead. Rationale:

1. This does not handle the "secondary" logo at all ie. breaks the parameter.
2. The raw SVG logo data will be injected into every HTML file, therefore will not be cached!