Refactor: generic name for components + langDir

[petersopko]

following from @roiLeo comment here:

• Folder structure #3068 (comment)

• name of our components is currently pretty random, if somebody with an eye for style guides could take this, it would be great.

• rename langDir folder to locales

• styles organisation: scss components, themes files // @roiLeo not sure what you meant by this part

ref:
https://v2.vuejs.org/v2/style-guide/?redirect=true#Multi-word-component-names-essential

[roiLeo]

styles organisation: scss components, themes files // @roiLeo not sure what you meant by this part

Theme file 🔽
In #2947, dark.scss & light.scss, we could extend for personalized chain theme

Scss components 🔽
Personal preference, I like to have components folder in scss to avoid polluting vue components code

And I'll move styles into assets folder (vue/nuxt good pratice)

[roiLeo]

Done in #3788 & #2947