Adapted Sib App for ETUC & more flexible way to handle optional components

Closes https://git.happy-dev.fr/startinblox/applications/sib-app/issues/377

Changed the way that we use the config.json to make every component (circles, projects) optional for ETUC (or any other client).

New config format:

{
  "xmpp": "https://jabber.happy-dev.fr/http-bind/",
  "authority": "http://localhost:8000/",
  "clientName": "Sib App",
  "clientLogo": "/images/logo.png",
  "clientLogoHeight": "32px",
  "clientCSS": "/path/to/custom.css",
  "authorityName": "djangoldp-server-name",
  "endpoints": {
    "get": {
      "circles": "http://localhost:8000/circles/",
      "projects": "http://localhost:8000/projects/",
      "customers": "http://localhost:8000/customers/",
      "users": "http://localhost:8000/users/"
    },
    "post": {
      "circles": "http://localhost:8000/circles/",
      "projects": "http://localhost:8000/projects/",
      "customers": "http://localhost:8000/customers/",
      "users": "http://localhost:8000/users/"
    }
  }
}

Unchanged: xmpp, authority, clientName, clientLogo, clientLogoHeight, authorityName

New:

• clientCSS (optional) An URL to a distant custom CSS for client.
• get & post properties (optional) For federation, fix for https://git.happy-dev.fr/startinblox/applications/sib-app/issues/377
• When an endpoint is missing, every related features will not be build with the application.

Minimal config.json become lightened too:

{
  "xmpp": "https://jabber.happy-dev.fr/http-bind/",
  "authority": "http://localhost:8000/",
  "endpoints": {
    "users": "http://localhost:8000/users/"
  }
}