mod_admin: Add button to run schema install from the site's module again

This is a very handy button when changing the datamodel of a site
frequently while developing the site. It runs
SiteModule:manage_schema(install, Context) site callback function, to
install the site's default data.
(cherry picked from commit 60f2c2ca9d1a660704b5461ce98c914407a35ec4)