Allow `myst build` without downloading web assets for low-internet situations

[choldgraf]

Currently when you run myst start, it queries a number of web-based sources for things like cross-references and embeds. This becomes a problem if you have really slow or non-existent internet (like on planes). Running myst start either hangs indefinitely, or results in an error like so:

Error loading template information from https://api.mystmd.org

This makes it impossible to preview MyST content locally until you get internet access again.

We should allow users to preview MyST sites using only local assets if possible, so that they aren't blocked by the lack of fast internet.

Ideas for solutions

We could have a --no-download flag that tells MyST not to attempt downloading anything from the web, and just get on with rendering a site locally for previews. This wouldn't work if the theme wasn't yet downloaded but that should be an acceptable edge case.