Specify that jquery slim cannot be used

There is a 'slim' version of jquery that apparently does not meet the requirements for mastodon-timeline-widget. It should be written in the docs to prevent people trying it.

Here's the error log when using https://code.jquery.com/jquery-3.3.1.slim.min.js to be more specific :

TypeError: $.ajax is not a function[En savoir plus] mastodon.widget.js:157:2