Support hosting at subpath

"In principle" setting `SCRIPT_NAME` is enough. The way we do this is [1] using
`FORCE_SCRIPT_NAME` (which does not depend on messing with reverse proxy
settings and [2] by deducing the correct value from `BASE_URL` (which must be
set anyway) automatically.

By works I mean: `reverse` and `{% url` pick it up from there.

However, there are subtleties / extra work:

* `STATIC_URL` is needed too b/c https://code.djangoproject.com/ticket/34028

* in many pre-existing code I just created a path manually in the html. Such
  hrefs are obviously not magically fixed for script_name. Rather than doing
  the "full rewrite" (into `{% url`) this commit just prepends the
  `script_name` in those cases. That's the way forward that will least likely
  break and it gives us something to grep for if we ever want to 'do it
  right'.

* `LOGIN_REDIRECT_URL` and `LOGIN_URL` needed to use a view-name for this to
  work (using a view-name gets revolved using the thing that introduces
  `script_name`)

Checked, no work needed:

* views (`redirect` and `HttpResponseRedirect`)
* html uses of action="..."

Fix #93

projects/templates/projects/project_list.html

@@ -64,7 +64,7 @@
             <td class="w-full p-4">
                 <div>
                     {% if project.member or request.user.is_superuser %}
-                    <a href="/issues/{{ project.id }}" class="text-xl text-cyan-500 dark:text-cyan-300 font-bold">{{ project.name }}</a>
+                    <a href="{{ script_prefix }}/issues/{{ project.id }}" class="text-xl text-cyan-500 dark:text-cyan-300 font-bold">{{ project.name }}</a>
                     {% else %}
                     <span class="text-xl text-slate-800 dark:text-slate-100 font-bold">{{ project.name }}</span>
                     {% endif %}