enhance(webhook): add select all and deselect all buttons for custom events (#35980)

Adds `Select All` and `Deselect All` buttons to the custom events
section of the webhook form. This is useful when you need all events but
one.

- The buttons toggle every event checkbox and keep the unsaved-changes
prompt working
- Fix the "Trigger On" radio spacing by removing a leftover Fomantic
checkbox margin

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/checkbox.css

@@ -88,10 +88,6 @@ input[type="checkbox"]:indeterminate::before {
 .ui.checkbox label {
   position: relative;
   display: block;
-}
-
-.ui.checkbox label,
-.ui.radio.checkbox label {
   margin-left: 20px;
 }
 
@@ -107,10 +103,6 @@ input[type="checkbox"]:indeterminate::before {
   pointer-events: none;
 }
 
-.ui.radio.checkbox {
-  min-height: var(--checkbox-size);
-}
-
 /* "switch" styled checkbox */
 
 .ui.toggle.checkbox {