Fixes extension icons not loading.

Might fix #1181, although not for the reasons I initially
thought (because the URLs are resolved from the manifest path, not the
path of the current page). This should ensure that the URLs used by the
manifest are always correct regardless of the manifest's path.

Also add a way to force a check.

Just a login form and a list of applications. No modals or anything like
that.

Now it should work whether you have a trailing slash or not.

Reworked from d5740128 to fit in the
new structure.

Otherwise there is risk of an infinite loop through the iframe where the
fallback keeps loading the root HTML which itself has an iframe...

This is mostly for development where VS Code might not have finished
compiling yet.

Revert "Decomission Travis"