fix(gui): get version from header when not authenticated (#9724)

### Purpose

Since #8757, the Syncthing GUI now has an unauthenticated state. One
consequence of this is that `$scope.versionBase()` is not initialized
while unauthenticated, which causes the `docsURL` function to truncate
links to just `https://docs.syncthing.net`/, discarding the section
path. This currently affects at least the "Help > Introduction" link
reachable both while logged in and not. The issue is exacerbated in
https://github.com/syncthing/syncthing/pull/9175 where we sometimes want
to show additional contextual help links from the login page to
particular sections of the docs.

I don't think it's any worse to try to preserve the section path even
without an explicit version tag, than to fall back to just the host and
lose all context the link was attempting to provide.

### Testing

- On commit b1ed2802fb (before):
  - Open the GUI, set a username and log out.
- Open the "Help" drop-down. The "Introduction" item links to:
https://docs.syncthing.net/
  - Log in.
- Open the "Help" drop-down. The "Introduction" item links to:
https://docs.syncthing.net/v1.27.10/intro/gui
- On commit 44fef317800ce1d0795b4e2ebfbd5e9deda849ef (after):
  - Open the GUI, set a username and log out.
- Open the "Help" drop-down. The "Introduction" item links to:
https://docs.syncthing.net/intro/gui
  - Log in.
- Open the "Help" drop-down. The "Introduction" item links to:
https://docs.syncthing.net/v1.27.10/intro/gui

### Screenshots

This is a GUI change, but affecting only URLs in the markup with no
visual changes.


### Drawbacks

If a `docsURL` call generates a versionless link to a docs page that
doesn't exist on https://docs.syncthing.net - presumably because
Syncthing is not the latest version and links to a deleted page? - then
this will lead to GitHub's generic 404 page with no link to the
Syncthing docs root. Before, any versionless link would also be a
pathless link, leading to the Syncthing docs root instead of a 404 page.

gui/default/syncthing/core/syncthingController.js

@@ -16,6 +16,15 @@ angular.module('syncthing.core')
             LocaleService.autoConfigLocale();
 
             if (!$scope.authenticated) {
+                function setVersionFromHeader(_data, _status, headers) {
+                    var version = headers('X-Syncthing-Version');
+                    if (version) {
+                        $scope.version = { version: version };
+                    }
+                }
+                // Get index.html again (likely cached) to retrieve the version header
+                $http.get('').success(setVersionFromHeader).error(setVersionFromHeader);
+
                 // Can't proceed yet - wait for the page reload after successful login.
                 return;
             }