Update stylesheets with a `POST` request

[dfabulich]

We currently change stylesheets with a GET request, which is a bad practice. (It might be possible to trick people into clicking on a link to change their stylesheet to one controlled by an attacker…?)

[salty-horse]

I have a WIP here that needs fixing of the CSS and HTML. I'll let you pick it up because I don't know the best way to fix this, and you have more experience.

Instead of a dedicated delete confirmation parameter, it now uses GET to show the confirmation page, and POST to do the deletion.

There was also an inconsistency between the wording of "Set as my active style" in the stylesheet gallery, and the page with the user's stylesheets.

Issues with my WIP:

1. Forms can't be inside span - should the form be outside? Or should we just use form-less buttons that POST the data with Javascript)
2. The button text doesn't look like the other links.