BExt: We don't have an “Open on Sourcegraph” button on “edit file” pages of GitHub

[vdavid]

Example page: https://github.com/sourcegraph/sourcegraph/edit/main/doc/dev/how-to/add_graphql_query.md

It'd make sense to add the "Open file on Sourcegraph" button to these pages as well.

In my particular case, I wanted to open the file on Sourcegraph while editing it to get more context about the file. (I think I wanted to browse the folder where this doc was located.) I assume that the more we become a code intelligence platform, the more often our users will want to do this.

/cc @muratsu @jjinnii @ryankscott