js: Set 'LINK_SUFFIX'

Sphinx 3.0.0 changed the behavior of JavaScript-based search utility,
changing the configuration option used to determine the suffix for
search results from 'html_file_suffix' to 'html_link_suffix' [1]. We
were manually overriding 'DOCUMENTATION_OPTIONS' and were not setting
the 'LINK_SUFFIX' attribute, which maps to this config option, meaning
our resulting URLs looked like 'fooundefined', where 'foo' is the name
of the file.

Add 'LINK_SUFFIX', resolving this issue.

[1] https://github.com/sphinx-doc/sphinx/commit/a717ffec78

Change-Id: I06488225cf0431e601f16e0f1047a6088a11d5bd
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>
Closes-bug: #1880331

openstackdocstheme/theme/openstackdocs/script_footer.html

@@ -21,6 +21,7 @@
         VERSION:     '{{ version }}',
         COLLAPSE_INDEX: false,
         FILE_SUFFIX: '.html',
+        LINK_SUFFIX: '.html',
         SOURCELINK_SUFFIX: '.txt',
         HAS_SOURCE:  true
       };