43 browser extensions 5

The chrome extension code can probably be merged now.

There are still some outstanding annoyances.

For instance, 'chrome-extension://' URLs aren't making it through webassets. It would be great if we could specify the extension URL (since we know its key after uploading) as the static asset url prefix. As it stands now, the best solution for production is to use https://static.hypothes.is.

Anyway, I think this is good enough to include for now. I added basic instructions to the readme.