reddit header: format karma number

In the top-right of every logged-in reddit page, we display the user's link
karma.  This number has been unformatted, which makes it a bit difficult to
read at a glance as you gain a fair amount of karma.  Now we'll format it with
periods or commas, according to the locale.

r2/r2/templates/redditheader.html

@@ -23,7 +23,12 @@
 <%!
    from pylons import request
    from r2.config import feature
-   from r2.lib.template_helpers import add_sr, display_link_karma, header_url
+   from r2.lib.template_helpers import (
+       add_sr,
+       display_link_karma,
+       format_number,
+       header_url,
+   )
    from r2.models import Sub, FakeSubreddit
    from r2.models.subreddit import DefaultSR
    from r2.lib.pages import SearchForm
@@ -78,7 +83,7 @@
     %else:
       <span class="user">
          ${plain_link(c.user.name, "/user/%s/" % c.user.name, _sr_path=False)}
-         &nbsp;(<span class="userkarma" title="${_("link karma")}">${display_link_karma(c.user.link_karma)}</span>)
+         &nbsp;(<span class="userkarma" title="${_("link karma")}">${format_number(display_link_karma(c.user.link_karma))}</span>)
       </span>
 
       ${separator("|")}