Move the BUR help text from the <textarea> placeholder attribute to a
<details> tag that embeds the [[help:bur_notice]] wiki page.
Also update some CSS for the <details> tag that was meant to only apply
to the user upgrades page and didn't look good here.
Refactor CSS to use standard Tailwind-style utility classes instead of
ad-hoc rules. This eliminates a lot of single-purpose rules for specific
UI elements and standardizes margins to be more consistent throughout
the site.
Utility classes are defined manually on an as-needed basis instead of
importing Tailwind as a whole. Naming conventions mostly follow
Tailwind's conventions, otherwise they follow Bootstrap.
* https://tailwindcss.com/docs/
* https://getbootstrap.com/docs/5.0/utilities/spacing/
* Add a frequently asked questions section.
* Add nicer looking upgrade buttons.
* Format the page nicer.
* Prevent users from attempting invalid upgrades on users that are
already Platinum or above.
Fineprint text was variously styled with `.info`, `.tn`, `.hint`, or
`.cost-footnote` css classes. Standardize on `.fineprint` instead. Use
`.hint` only for form hints and `.tn` only for <tn> tags in translation
notes.
Incidentally changes the font size of form hints to 0.8em (was 0.7em)
and the color of fineprint to #888 (was #AAA or #666).
