_s: Simplify entry meta output

- Fix entry-footer inconsistency between content-*.php files
- Move in a function <code>_s_entry_meta()</code> to avoid duplication

---

At the moment there is an inconsistency between content.php and
content-single.php regarding the .entry-footer output.

In my opinion content.php output is would be better to use because it's
easier to translate and allow more customisation.

Finally by moving it into <code>inc/template-tags.php</code> we are
getting rid of some duplicated code.

content-page.php

@@ -20,7 +20,8 @@
 			) );
 		?>
 	</div><!-- .entry-content -->
+
 	<footer class="entry-footer">
-		<?php edit_post_link( __( 'Edit', '_s' ), '<span class="edit-link">', '</span>' ); ?>
+		<?php _s_entry_meta(); ?>
 	</footer><!-- .entry-footer -->
 </article><!-- #post-## -->