Improves performance and more

[Berkmann18]

This PR initially started as a performance-focused improvement plan.
I also got around to improving (at least according to the local LightHouse evaluations I did) the app in a11y (accessibility), best practices, SEO and progressiveness.
There's still room for improvement in performance, best practise and in progressiveness/offline-friendliness tho, I listed some ideas at the bottom of this PR.

The LH score here (as well as the one on handsdown.dev will possibly be a bit different in terms of results) were:
Given the following format: Performance / A11y / BestPractices / SEO / (PWA: Fast&Reliable, Installable, PWA Optimised) = AverageScore % where all but the PWA metrics are in % and the PWA ones are respectively out of 3, 3 and 7 (this is just to simplify the results).

• Before: 13 / 94 / 86 / 75 / (0 / 0 / 2) = ~42.37% (as of 2653c53)
• Now: 27 / 100 / 86 / 100 / (0 / 1 / 6) = 61.72% (as of 4f5184d)

Re #7

Some other ways to improve the performance include:

• Boot-up / loading:
  • Bundling (@amirhmk is working on this afaik).
  • File minification (currently only the vendor CSS and 3rd party JS code are).
  • Eliminating dead code.
• Post-FMP / Post-Boot-up:
  • DOM
    • Using either a Shadow or Virtual DOM approach (either in-house or via a library/framework like Mithril/Snabbdom/Pureact/Preact+HTM/MaquetteJS/Stage0/Cycle.js/O!/...).
    • Reducing the amount of DOM manipulation (can be dealt with using a VDOM).
    • Using DocumentFragment where possible.
    • Using DOM creation utilities like crel.
  • Computations.
    • Dealing with only as many frames as needed (@misterpeddy mentioned that it instead of dealing with 60fps, the model could only deal with 20-25fps).
    • Short-circuiting whenever possible (Improves extrapolation #49 helps with that).

Edit: The LHCI score change is as follows:

Routes	Perf	A11y	BP	SEO	PWA	Average (in %)
index@master	58	94	79	60	1 / 0 / 1	48.37
index	83	100	79	100	1 / 1 / 6	73.48
∆	+25	+6	0	+40	0 / +1 / +5	+25.11
dev@master	36	100	79	60	1 / 0 / 2	48.13
dev	79	98	79	100	1 / 1 / 6	72.63
∆	+43	-2	0	+40	0 / +1 / +5	+24.5