View Submodule may fail for github-fetched modules

[dehall]

When you open a module from a github branch, and click View Submodule, it tries to open the submodule by URL in a new window, which fails when the referenced submodule isn't in the builtin version of the modules.

We should
a) open a new tab within the app instead of a new window
b) fetch from the same github branch as the current module, when appropriate