Use an <img> for SVG output instead of an <object>

[tsibley]

<img>, unlike <object>, has proper alt text support (i.e. that's not just fallback content) and is easily saved, opened in a new tab, etc. from the page where it appears.

This does mean the SVG's DOM will no longer be available for inspection by JavaScript in the containing document (e.g. object.contentDocument), but I don't think that was intended to be a feature of SVG output for this Sphinx extension.

Resolves #102 as well.

[jougs]

One possible problem with this is that using <img> instead of <object> makes it impossible to select text in the figure and click on embedded links, which are both things I use a lot and would not want to miss.

Maybe this could be made configurable via a setting in conf.py? Something like mermaid_inline_svg = True | False would probably be a good compromise.

[tsibley]

@jougs Noted! One thing I'll point out (in case it's not clear) is that the change in this PR only applies when the SVG is generated at build time instead of dynamically on page load, and build-time generation is not the default mode of operation for this Sphinx extension.

Making it configurable should be no problem, though, and sounds worthwhile. With mermaid_inline_svg = True, I'd expect that instead of emitting an <img> tag pointing at the external SVG file generated at build time, the contents of the SVG file would be emitted directly instead (sans doctype, if present). So it'd still not be using <object>. Does this match your expectation?

That said, I'm unlikely to actually do more work on this PR until @mgaitan or another maintainer of this project gives the nod that they're interested in accepting and releasing it.