Check for correct permissions

Originally, the gym models resided in the core app but where moved to their own
'gym' app later.
Small fixes to permission checks for new gym users

Gym managers can edit their own gym only, general gym managers
can edit all gyms
Add fixtures for gym and associated configurations

While this is not actually needed, it makes the admin user in new installations
more useful.
Mention gym management in README
Update log screenshot for feature page
Show navigation on feature page for users

With this change, new visitors that arrive at the landing page get a very
clean page, with no distracting menus. Logged in users can still reach all
the usual navigation links.