refactor: Replace boostrap with custom CSS

[MoritzWeber0]

Bootstrap was a good library to get started faster, but we've mostly used custom styling and most of the bootstrap functionality wasn't used.

Removing bootstrap provides a few advantages:

• We gain full control over the styles
• Better adoption to CSS BEM (we can choose our own class names)
• Reduced bundle size
• Better DevTools support. Chrome and Firefox have some neat development tools around grid layout. The devtools didn't work with the bootstrap grid layout.
• Less chance of breaking changes. Bootstrap is under active development and can introduce breaking changes. CSS itself is considered as rather stable.

As proposed in #192 (comment), the removal of bootstrap as separate PR. Resolves #74.

Beside some minor changes to margins and sizes, the most significant visual change is the style of tables, which I changed as part of this PR.

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[therobrob]

Thanks @MoritzWeber0 for your effort removing bootstrap! <3