Move frontend from Bootstrap to Bulma and remove jQuery; dark theme support; some additional features

[Merlin04]

Hi, and thanks for the wonderful project - I realize this is a pretty big PR to make without any prior discussion, so please don't feel any obligation to work on this or merge this if it wouldn't be helpful to bepasty! I made these changes for my own personal instance of bepasty (https://paste.tail917c8.ts.net) and I figured I might as well open a PR in case you all might want to merge them.

Screenshots

Architectural changes

• Migrate frontend from Bootstrap to Bulma, removing jQuery along the way
  • This allows for dark theme, reduced network/JS payload, and removal of legacy dependencies
• Replace jQuery file uploader with FilePond, an MIT-licensed async chunking file upload widget
• replace jQuery-based carousel with fslightbox.js

New features

• Dark theme! Uses the prefers-color-scheme CSS media query to detect user's preference. pygments uses the monokai theme when dark theme is active
• You can now click a button to instantly undo a binary file upload

• New "Copy URLs" feature lets you copy a newline-separated list of urls to uploaded files (inline or preview). This is great for situations where the list feature is overkill, like if you're uploading a few media files to share with someone on a chat app

Regressions/possible issues

• Some libraries are being loaded from a CDN because there isn't an xstatic package for them. I'm not super familiar with xstatic or the python packaging ecosystem in general, so maybe there's a better way to do this?
  • specifically, that's bulma.min.css, fslightbox.js, and filepond.js
• Carousel takes a slightly different format - it's now images on a page, and when you click one a full screen lightbox carousel view opens. I prefer this over the previous behavior, but I'm unsure if others would given how different it is from previous behavior
  • I've been thinking about possibly switching from fslightbox to lightgallery.js or photoswipe. Both of these libraries would provide a much nicer looking gallery view that puts thumbnails in a grid; however, they require knowing the resolution of the images, so I would have to add a function to the backend to calculate this (with similar logic to the thumbnail generator)
• Use of some inline style tags in templates instead of a single CSS file when they only apply to UI components on a single page - I'm unsure if you have a preference for one or the other, but I didn't see many uses of inline style tags in the codebase so I figure I should bring this up
• I removed the documentation link from the navbar and adjusted the index page shown to logged-out users a bit to match my personal preference - I like it better this way but that's just personal preference

WIP/TODOs

• Tests haven't been updated to match new element names (this is why the PR is a draft)
• AUTHORS and CHANGES.rst have not been updated

[Merlin04]

@ThomasWaldmann Hi, hope you're doing well! I have a bit more time now and might be able to work on this PR more, so I wanted to ask - are you interested in the possibility of merging this, or would you prefer it to remain a fork? If the former, I could try to finish up the remaining bits of the PR so it can exit draft stage.