Feat(uiux): integrate SCSS from Bootstrap on Nuxt/Vue

[xinbenlv]

Currently, we are using bootstrap-vue, but we have not leveraged the SCSS power of it.
We are instead doing one-off importing for the variables from bootstrap scss, e.g.

<style lang="scss" scoped>
@import 'bootstrap/scss/_functions.scss';
@import 'bootstrap/scss/_variables.scss';
@import 'bootstrap/scss/_mixins.scss';
// ... definitions
</style>

Our goal is to

1. allow using the scss variables/mixins defined from Bootstrap, such as @include media-breakpoint-up without needing to import everything.

2. allow customizing certain behavior from original Bootstrap, such as
   a. change what's success color, what's danger color
   b. change the default font-size
   c. change the default outline color.

3. It also allows us to apply different color theme, font, etc.

Reference:

1. https://getbootstrap.com/docs/4.5/getting-started/theming/#sass
2. https://medium.com/javascript-in-plain-english/customize-bootstrap-in-nuxt-3da863703b35
3. https://bootstrap-vue.org/docs#nuxtjs-module