fix(usermanager): disable duplicate ajax requests

- disable `.toggleMembershipButton` buttons while ajax request is in
  progress

lumi2/static/js/group_edit.js

@@ -65,6 +65,7 @@ class UserEntry {
             '<span class="spinner-border spinner-border-sm" role="status" aria-hidden="true"></span>' + 
             '<span> Loading...</span>'
         );
+        this.buttonElement.prop("disabled", true);
     }
 
     setButtonAppearanceLeaveGroup() {
@@ -74,6 +75,7 @@ class UserEntry {
         this.buttonElement.html(
             '<i class="bi-person-fill-dash"></i> Remove'
         );
+        this.buttonElement.prop("disabled", false);
     }
 
     setButtonAppearanceJoinGroup() {
@@ -83,6 +85,7 @@ class UserEntry {
         this.buttonElement.html(
             '<i class="bi-person-fill-add"></i> Add'
         );
+        this.buttonElement.prop("disabled", false);
     }
 }