Allow translation of text specified in template file

This patch includes the following fixes:

• Added appropriate calls to t() and format_plural() to make strings translatable in og-extras-group-info.tpl.php.
• Used format_plural() instead of t() in function og_extras_preprocess_og_extras_group_info() so that the appropriate singular or plural form is selected when displaying the member count.
• Fixed bad path in member count link. This has been reported in #1711818: Group Details block has link with space, but I have included the fix here since the patch was already modifying the same line.