Allow the bug title to be customisable

Some projects using openstackdocstheme may wish to customise
the default bug title to be better suited to their needs.

This patch implements the ability to do so.

To define it, add the variable to html_context in the Sphinx
conf.py.

For example:
  html_context = {"bug_title": 'Documentation bug', ...}

Change-Id: Id62debd50fe0499ace809dde626cd8a357dbf933

openstackdocstheme/theme/openstackdocs/script_footer.html

@@ -41,7 +41,11 @@
     {%- else %}
         var bugProject = "openstack-manuals";
     {%- endif %}
-    var bugTitle = "{{ title }} in {{ project }}";
+    {%- if bug_title %}
+        var bugTitle = "{{ bug_title }}";
+    {%- else %}
+        var bugTitle = "{{ title }} in {{ project }}";
+    {%- endif %}
     var fieldTags = "{{ bug_tag }}";
 
     /* "last_updated" is the build date and time. It relies on the

releasenotes/notes/bug-title-fdbefea0408e2cbf.yaml

@@ -0,0 +1,13 @@
+---
+features:
+  - |
+    The ability to customise the bug title for the 'Report a Bug'
+    link is now available. To customise the bug title used add
+    the ``bug_title`` key with a value to ``html_context`` in the
+    Sphinx configuration.
+
+    For example:
+
+    ::
+
+      html_context = {"bug_title": 'Documentation bug', ...}