Closes #100

This is based very closely on the theme-switching UX.

Set up: add "argus_htmx.context_processors.datetime_format_via_session" to the context_processors" in the TEMPLATES-setting in your localsettings-file, or change the argus_htmx-entry in EXTRA_APPS to:

  {
    "app_name": "argus_htmx",
    "urls": {
      "path": "",
      "urlpatterns_module": "argus_htmx.urls"
    },
    "context_processors": [
      "argus_htmx.context_processors.theme_via_session",
      "argus_htmx.context_processors.datetime_format_via_session"
    ],
    "middleware": {
      "argus_htmx.middleware.LoginRequiredMiddleware": "end"
    }
  },

One difference from the themes: in order to actually switch to the new format, the page is always refreshed. There might be redundant htmx-code for that reason.

There's a new optional setting ARGUS_FRONTEND_DATETIME_FORMAT that sets the default format. If not set i fall backs to using locale.