Make doc footer configurable

This will let custom templates override the default doc footer to put
site-specific links.

Change-Id: I46be380d8a1760c8d67f0200e0ba50462fb49bbf

resources/com/google/gitiles/templates/Doc.soy

diff --git a/resources/com/google/gitiles/templates/Doc.soy b/resources/com/google/gitiles/templates/Doc.soy
index d1c0f79..561e9d0 100644
--- a/resources/com/google/gitiles/templates/Doc.soy
+++ b/resources/com/google/gitiles/templates/Doc.soy
@@ -14,6 +14,27 @@
 {namespace gitiles}
 
 /**
+ * Default Doc Footer
+ *
+ * @param? sourceUrl url for source view of the page.
+ * @param? logUrl url for log history of page.
+ * @param? blameUrl url for blame of page source.
+ */
+{deltemplate gitiles.customDocFooter}
+<!-- default customDocFooter -->
+<footer class="Site-footer">
+  <div class="Footer">
+    {delcall gitiles.footerPoweredBy /}
+    <div class="Footer-links">
+      {if $sourceUrl}<a class="Footer-link" href="{$sourceUrl}">{msg desc="text for the source link"}source{/msg}</a>{/if}
+      {if $logUrl}<a class="Footer-link" href="{$logUrl}">{msg desc="text for the log link"}log{/msg}</a>{/if}
+      {if $blameUrl}<a class="Footer-link" href="{$blameUrl}">{msg desc="text for the blame link"}blame{/msg}</a>{/if}
+    </div>
+  </div>
+</footer>
+{/deltemplate}
+
+/**
  * Documentation page rendered from markdown.
  *
  * @param? siteTitle h1 title from navbar.md.
@@ -21,9 +42,6 @@
  * @param? logoUrl url of image logo.
  * @param? homeUrl url to jump to top of site.
  * @param? analyticsId Google Analytics Property ID.
- * @param? sourceUrl url for source view of the page.
- * @param? logUrl url for log history of page.
- * @param? blameUrl url for blame of page source.
  * @param? navbarHtml navar.md converted to SafeHtml.
  * @param? customVariant variant name for custom styling.
  */
@@ -64,16 +82,7 @@
       </div>
     </div>
   </div>
-  <footer class="Site-footer">
-    <div class="Footer">
-      {delcall gitiles.footerPoweredBy /}
-      <div class="Footer-links">
-        {if $sourceUrl}<a class="Footer-link" href="{$sourceUrl}">{msg desc="text for the source link"}source{/msg}</a>{/if}
-        {if $logUrl}<a class="Footer-link" href="{$logUrl}">{msg desc="text for the log link"}log{/msg}</a>{/if}
-        {if $blameUrl}<a class="Footer-link" href="{$blameUrl}">{msg desc="text for the blame link"}blame{/msg}</a>{/if}
-      </div>
-    </div>
-  </footer>
+  {delcall gitiles.customDocFooter variant="$customVariant ?: ''" /}
   {if $analyticsId}
     /* From https://developers.google.com/analytics/devguides/collection/analyticsjs/ */
     <script>