MDL-38509 Add hook for the installaddonrequest into admin/index.php
If the user selects a plugin in Moodle plugins directory to be installed
into their site, Moodle plugins directory will redirect them into their
site's admin/index.php script, providing the installaddonrequest
parameter.
This patch makes sure that only if the user is logged-in as the admin
and the site is fully installed and up-to-date, the add-on installation
request will be dispatched to the tool_installaddon for actual
processing.
We need to store the installaddonrequest value in the $PAGE's URL so
that is is stored in $SESSION->wantsurl in case the user needs to log in
at their site. Thanks to this, the request is dispatched after the user
logs in.
