Messages: add support for paginating thread messages (in web version)

[espellcaste]

We added support for paginating thread messages at #9146 and #8597, but this only takes into account the BP REST API, not the web version which loads all private messages by default.

We discussed ​before about adding a theme support, so that we don't break the experience for themes that rely on the legacy experience.

add_theme_support(
    'buddypress',
    [
        'messages' => [ 'pagination' => true ]
    ]
);