Remove Bootstrap and jQuery dependencies

[Alanaktion]

They're awesome for prototyping, but are unnecessarily heavy and inflexible for our cases. This change will be done over a fairly long time and will be turned into a milestone with separate issues for each change eventually.
I'd like to move everything to a vanilla ES2015/SCSS setup, using only small helper libraries like Underscore.js and Reqwest.js (see vanillalist.com). Bootstrap 4's functionality is great, but we'll never use 80% of what it does, so we're better off building our own minimal UI library in SCSS.
Also, it should srsly be more semantic. No more stupidly verbose classes, let's take advantage of CSS's cascading aspect!

[Alanaktion]

Current compiled CSS sizes:

• Normalize: 3 KB
• Bootstrap v4 Alpha 2: 94 KB
• Icomoon custom: 13 KB
• Vault styles: 5 KB

Dropping Bootstrap would be nice for sure filesize-wise. Keeping normalize.css is probably a good idea, if there's a SCSS port we should use that. The current Icomoon font should be replaced with a new one using ligatures so the CSS is much simpler.