Remove jQuery migrate

[onli]

Removes jQuery migrate and replaces the parts of the backend JS that does not work without it.

A followup to #687

[onli]

Done in the backend as far as I can see. Edit: No, the tabs would be better when implemented with detail/summary instead. I'll test that out at least.

TODO: Test plugins and themes as in #686 (comment)

Core themes that use jQuery

• clean-blog
• next
• default (I think, I'm not 100% sure)
• 2k11, including in the admin interface
• bootstrap4 (but that should work with jQuery 3.x since bootstrap bundles that)
• timeline

The biggest potential issue I have seen here so far is a dependency on FitVids, which requires jQuery, in clean-blog, timeline and 2k11, which is a third-party script that hasn't been updated in a long time. Then there's of course our backend JS, which uses jQuery and also uses plugins that depend on jQuery.

^-- TODO: Check the FitVid part, I'm not aware of that dependency. I only checked clicking visiting entries with the themes.

It seems like no core plugin uses jQuery.

Additional plugins that require jQuery

• serendipity_event_cal
• serendipity_event_commentedit
• serendipity_event_emoticonchooser
• serendipity_event_faq 🚫 (see [serendipity_event_faq] Deprecation warnings #717)
• serendipity_event_freetag (had a bundled jquery.migrate, to be removed in freetags 4.0: Cleanup outdated options additional_plugins#180)
• serendipity_event_geshi
• serendipity_event_guestbook 🚫 (see [serendipity_event_guestbook] Installation fail #718)
• serendipity_event_karma
• serendipity_event_lightbox (depends on lightbox: Drop pretty photo & jQuery additional_plugins#179, lightbox part works already)
• serendipity_event_linktrimmer
• serendipity_event_livecomment
• serendipity_event_podcast
• serendipity_event_realtimecomments
• serendipity_event_social
• serendipity_event_spamblock_bayes
• serendipity_event_staticpage
• serendipity_event_template_editor
• serendipity_event_usergallery 🚫 (see [serendipity_event_usergallery] Error message when clicking image #722)