multiple StudyProgram admins support + HEI admin page + new landing site

- Added support for multiple StudyProgram admins. This required some
rewrites in the general flow of the application, mainly by implementing
a StudyProgram selection screen before accessing certain pages. This
selection screen will only appear if normal navigation is used.
- a person with role ROLE_HEI_STAFF_ADMIN can access the admin page,
but adding new StudyPrograms has been disabled.
- The indexAction now redirects to the a /static/ folder in the /web
directory. A rule has been added to the .htaccess file in the /web
directory to allow this folder to serve pages normally.