Moves admin dashboard init code from adminvitals.js to admincore.js

Fixes bug introduced in commit f0f5c97cc2, which moved code from the
dashboard to adminvitals. This broke the rest of the dashboard
(i.e. adminclass and any future dashboard modules) if adminvitals
wasn't enabled. This commit makes a new file, admincore.js,
which is imported from the dashboard mainpage.