Use Vue 3 in the frontend

[ninoseki]

Vue v3 has released and it provides improved performance, smaller bundle sizes, etc.
https://github.com/vuejs/vue-next/releases/tag/v3.0.0

I think it is a good time to revamp the frontend.
I will work on this if it makes sense. with you @Fitblip.

[Fitblip]

Hi @ninoseki, if you want to go for it, though I think it works fine as-is. Smaller bundles are always a plus though :)

[ninoseki]

@Fitblip I compared bundle sizes of the current Vue2 implementation and my Vue3 implementation.

Vue2

$ du -k dist/
924     dist//static/css
1384    dist//static/js
2624    dist//static/img
1600    dist//static/fonts
6556    dist//static
6560    dist/

Vue3

$ du -k dist/
372     dist//static/css
1564    dist//static/js
2732    dist//static/img
1644    dist//static/fonts
6312    dist//static
6316    dist/

The difference is not bigger than I expected but the Vue3's bundle size is smaller than the Vue2.

Also, I'd like to mention that Vue3's configuration files are more easy to understand than the current version. It's an advantage.