Improved category select on new thread creation

Not fixed but replaced with new start thread flow which first ask user to pick a category in which they want to start new thread in:

This flow only happens on site-wide threads lists. Inside a category "Start thread" button points directly to a new thread form for current category. And, if there is no permission to start new thread in a category, one of two things happens:

• If you are signed in, you don't get to see the "Start thread" button.
• If you are not signed in, depending on "start threads" permission in current category, you either see "Start thread" button that will first point you at sign in page and then redirect you to new thread form, or you don't see the button.

For your codebase it would be faster* to just modify current "start thread" logic in React.js code. In 0.40 there is no React.js on threads list anymore and this button in stupid bootstrap modal + htmx for loading its contents.

*assuming you will outpace me and get your fork running on 0.39 before this year ends.