Removing page template and cleaning up code

[rugk]

The problem

Upgrade to Bootstrap 5 in #1287 has shown:

• we have a big maintenance burden by maintaining the page template
• we have to care when adjusting JS to not break all the templates and test with all of them
• when we adjust the templates, in reverse, we have to care for the JS and cannot easily change the HTML structure yet again and adjust it to some modern one etc.

The solution

• first of all: remove the page template altogether
• later/maybe not yet(?): remove the old bootstrap template
• rename CSS classes/unify them
• use proper/modern HTML tags where useful, like <template>
• adjust the JS if needed
• remove outdated JS code only useful for older bootstrap versions etc.
• add the CSS classes intended for boootstrap 5/adjust and remove custom CSS styling if needed
• fix some accessibility issues like for alerts and buttons.

Alternatives

• keep maintaining stuff, the desire does not seem to be big though

Additional context

[elrido]

For what it's worth, we do not "have a big maintenance burden by maintaining the page template", IMHO. It doesn't use any extra JS-libraries and extra code paths in JS like the bootstrap 3 template does. Removing the bootstrap 3 template is in my opinion a way higher priority (especially with it's JS lib being flagged as having security issues) and I gave a suggested timeline for doing that. I'm not aware of any JS code we could remove when deprecating the page template.

It is (subjectively) ugly and we may want to remove backwards compatibility to ZeroBin and that would reduce JS and PHP complexity for all templates, but those are not related to the page template, but affect all of them.

I would strongly suggest, that if we remove this, we do so when another non-bootstrap template gets introduced, so that we have at least 2 templates available and can therefore ensure that our JS code remains template agnostic.