Focus highlight is not sufficient on topbar notifications button

This is because of the bootstrap class styles that are not aligned with what we want:
https://github.com/xwiki/xwiki-platform/blob/da40079fdb99261269f8724f74f93f06da357116/xwiki-platform-core/xwiki-platform-bootstrap/src/main/less/dropdowns.less#L24-L27

There are two solutions:

• Override the default style like it's done for the breadcrumb dropdown toggles (like it's done in https://github.com/xwiki/xwiki-platform/blob/35425fd9bd4dad979adb5b834770fddb5e3eb734/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/breadcrumbs.less#L42-L46 ).
• Change the default style.

The second one is probably the cleaner long term, but it could create complications because this class is used in quite a few places in XWiki.
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I'll go for a compromise: option 1 but add a comment on the default code. The comment will point out that we already hard-code an override to this in two places, next time we have trouble with it we probably want to change it