[Refactor] split rootstore

[yuvalbl]

Blocked by #843

See combining-multiple-stores

Root store is too big, we should split it as follows (store + fields):

userStore (userStore.tsx):

• isUserAuthenticated
• userApiError
• userInfo

newsFlashStore (newsFlashStore.tsx):

• activeNewsFlashId
• newsFlashFetchOffSet
• newsFlashActiveFilter
• newsFlashLoading

widgetStore (widgetStore.tsx) :

• newsFlashWidgetsMeta (rename to widgetsMeta)
• newsFlashWidgetsData (rename to widgetsData)
• widgetBoxLoading (rename to widgetsLoading)

move language properties (like currentLanguageRouteString and selectedLanguage) to settingStore:

IMPORTANT:
after done splitting, make sure you remove the /* eslint-disable*/ comment from the root store, to allow linter to scan this file as well.