Install Firefox Webapp from local checkout

Ask Firefox to install the currently running copy of glowing bear instead of the copy from torhve.github.io

This also fixes a bug when glowing bear is not running from a subdirectory (or one other than /glowing-bear) which would lead to a 404 because of torhve.github.io/original_subdir being loaded instead of /glowing-bear.

Note that installing to a different subdirectory than /glowing-bear will lead to the icons not being found due to the fact that all icon paths inside manifest.webapp [have to be absolute](https://developer.mozilla.org/en-US/Apps/Developing/Manifest#icons).

I can't think of a generic solution for the latter other than maybe using a `.htaccess` rule like

    RewriteRule ^glowing-bear/(.*) $1

but that's of course also highly dependent on the user's setup.