Fix #905 — two strings got colons and one got %s.

WARNING WARNING WARNING

Custom themes (specifically themes that customize base.tmpl, index.tmpl,
post.tmpl or  post_helper.tmpl)  may  be  broken,  as  of  this  commit.

However, because we strive to retain backwards compatibility,  we  still
provide the old strings for those who  still  need  it.   They  will  be
removed in Nikola v7.0.0.

The following translations were changed:

 * Posted            → Posted:             [post.tmpl, index.tmpl]
 * Also available in → Also available in:  [base.tmpl]

   In order to fix those, just  change  the  messages()  call  in  those
   templates (if any — bootstrap themes, for  example,  don’t  need  the
   second patch).

 * More posts about  → More posts about %s [post_helper.tmpl]

   In order to fix this one, you need to do some more work.  A git patch
   depicting the change is reproduced below.  Old themes  will  work  up
   until v7.0.0.

diff --git a/nikola/data/themes/base/templates/post_helper.tmpl b/nikola/data/themes/base/templates/post_helper.tmpl
index f6b0535..4276a8c 100644
--- a/nikola/data/themes/base/templates/post_helper.tmpl
+++ b/nikola/data/themes/base/templates/post_helper.tmpl
@@ -18,15 +18,18 @@
     %endif
 </%def>

+<%def name="html_list_tags(post)" buffered="True">
+    <span itemprop="keywords">
+    %for tag in post.tags:
+        <a class="tag p-category" href="${_link('tag', tag)}"><span class="badge badge-info">${tag}</span></a>
+    %endfor
+    </span>
+</%def>

 <%def name="html_tags(post)">
     %if post.tags:
-        &nbsp;&nbsp;|&nbsp;&nbsp;${messages("More posts about")}
-        <span itemprop="keywords">
-        %for tag in post.tags:
-            <a class="tag p-category" href="${_link('tag', tag)}"><span class="badge badge-info">${tag}</span></a>
-        %endfor
-        </span>
+        &nbsp;&nbsp;|&nbsp;&nbsp;${formatmsg(messages("More posts about %s"),
+        html_list_tags(post))}
     %endif
 </%def>