gui: Specialize a special-purpose checkbox style (#9236)

### Purpose

Discovered while working on the WebAuthn credentials table in #9175:
there's a style on `td input[type="checkbox"]` that modifies margins for
all checkboxes in `<table>`s. It looks like this style is specially
tailored to the particular table that added it (PR #8734), so it should
have a correspondingly special-purpose class to not accidentally apply
it to other tables.

As best as I could tell there are only 2 instances of `<input
type="checkbox">` in `<td>`s, shown in the screenshots below.

### Testing

- Open "Actions > Logging > Debugging Facilities" and observe the
vertical spacing of the checkboxes.
- Open "Edit Folder > Advanced", check "Sync Extended Attributes" or
"Send Extended Attributes", click "Add filter entry" and observe the
vertical spacing of the checkbox that appears.

### Screenshots

#### Before

![Logs > Debugging
Facilities](https://github.com/syncthing/syncthing/assets/1367758/998fc66d-a0ad-41d9-a476-7a2b3da622d1)
![Add filter
entry](https://github.com/syncthing/syncthing/assets/1367758/647cb565-fcd0-4a81-a6ca-1f75137039b0)

#### After

Logs > Debugging Facilities now more compact:
![Logs > Debugging Facilities now
](https://github.com/syncthing/syncthing/assets/1367758/7cf8fc77-610e-4b4a-be21-c50d30be7bb9)

Add filter entry unchanged:
![Add filter
entry](https://github.com/syncthing/syncthing/assets/1367758/0ba710d6-cee1-49b4-92bc-acfc0c22c2bd)

gui/default/assets/css/overrides.css

@@ -143,7 +143,8 @@ table.table-auto td {
     max-width: 0px;
 }
 
-td input[type="checkbox"] {
+/* Tag name is needed for selector to be specific enough to override Bootstrap style */
+input[type="checkbox"].extended-attributes-filter-rule-checkbox {
     margin-top: 13px;
 }
 

gui/default/syncthing/folder/editFolderModalView.html

@@ -344,7 +344,7 @@
                 </colgroup>
                 <tr ng-repeat="entry in currentFolder.xattrFilter.entries">
                   <td>
-                    <input type="checkbox" ng-model="entry.permit">
+                    <input type="checkbox" ng-model="entry.permit" class="extended-attributes-filter-rule-checkbox"/>
                   </td>
                   <td><input class="form-control text-left" aria-required="true" ng-model="entry.match"/></td>
                   <td>