POC: be able to bypass permissions using an admin token

[alexAubin]

Soooo today I got fed up of seeing again stuff about having to "unprotect" an app to be able to run some curl requests, then having to protect-it-again-but-we-dont-really-know-if-it-was-protected-or-not-in-the-first-place

That turns a bunch of things that should have been simple into a complex mess.

So instead I'm investigating the idea of :

• being able to define an admin token to bypass permissions restrictions using a header, let's call it SSOwat-Admin-Token
• in ynh_local_curl, we temporarily add such a token and inject the corresponding header in the request
• no need to tweak permission before/after ynh_local_curl anymoar
• ???
• PROFIT!

To test this PR:

• Pull the corresponding branch, restart nginx
• install -m 400 -o www-data <(echo secret_token) /etc/ssowat/admin_token
• install an app in private mode. In my case I installed hextris
• try curling the page without any auth, this should show the ssowat portal title: curl -s -L https://yolo.test/hextris --insecure | grep title
• try adding --header "SSOwat-Admin-Token: secret_token"