invite: Fix label for new user's role dropdown.

This commit updates label for new user's role dropdown to use
"Users" instead of "User(s)" because that caused some problems
in translating the text.

We should ideally be using the ICU plural syntax for such cases,
but since it is used as label, we cannot use the plural syntax
there. So we instead just change it to always have "Users"
plural like we do at some other places in that modal.

web/templates/invite_user_modal.hbs

@@ -52,7 +52,7 @@
         </div>
     </div>
     <div class="input-group">
-        <label for="invite_as">{{t "User(s) join as" }}
+        <label for="invite_as">{{t "Users join as" }}
             {{> help_link_widget link="/help/roles-and-permissions" }}
         </label>
         <select id="invite_as" class="invite-as modal_select bootstrap-focus-style">