fix: back link on password page needs to be explicit.

The back link on the password page was using Javascript to tell the
browser to navigate back, which won't work if the user has entered a
set of incorrect log-in details.  Fix this by using an explicit URL
instead.

Fixes #1851

Signed-off-by: Alastair Houghton <alastair@alastairs-place.net>

web/templates/password.html

@@ -27,7 +27,7 @@
   </form>
   {{ if .BackLink }}
   <div class="theme-link-back">
-    <a class="dex-subtle-text" href="javascript:history.back()">Select another login method.</a>
+    <a class="dex-subtle-text" href="{{ .BackLink }}">Select another login method.</a>
   </div>
   {{ end }}
 </div>