Refractor primary nav and dashboard, hide projects info if not enabled (#3898)

Also, refractor and allow for importing permission functions from
anywhere and not just limit to be imported from the
`hypha.users.decorators` module

This PR also doesn't display project info on the dashboard if the
projects is not enabled

For the stats block at the top of the dashboard, if the projects is not
abled displays the count of flagged submissions and the count of
previously reviewed submissions

Simplify the nav generator logic.