core:  Close erlang port to bin/zmm

Also add method to build module path relative to current site's install path.
core: Add module update method
core: Add module uninstall method
mod_admin_modules: Add documentation
mod_admin_modules: Replace buttons with dropdown actions menu.

Attach custom server-side Zotonic actions to the actions menu items.
mod_admin_modules: Add custom Zotonic action module for module updates
mod_admin_modules: Add custom Zotonic action module for module uninstall
mod_admin_modules: Add custom Zotonic action module for module reinstall