[web] delegate authenticate outside of the app in embed mode

[LukasHirt]

Description

User Stories

• As a user, I want to authenticate only once in the main application so that I do not need to authenticate again inside the embed mode

• As a developer, I want to have an easy solution to disable the authentication flow in the embed mode so that I can provide the token on my own

• As a developer, I want to have an event that the embed mode will listen to so that I can provide the access token to it

• As a developer, I want to be able to fire the event via postMessage method so that it works across different origins

Value

• user is not forced to authenticate twice
• smooth flow of getting/saving resources from the embed mode

Acceptance Criteria

• login form is not shown to the user
• an event listener is created to listen to receive token (and refresh it)
• when a redirect happens, the embed=true query param is not lost
• the event to refresh token and config options are documented on owncloud.dev

Definition of ready

• Everybody needs to understand the value written in the user story
• Acceptance criteria have to be defined
• All dependencies of the user story need to be identified
• Feature should be seen from an end user perspective
• Story has to be estimated
• Story points need to be less than 20

Definition of done

• Functional requirements
  • Functionality described in the user story works
  • Acceptance criteria are fulfilled
• Quality
  • Code review happened
  • CI is green (that includes new and existing automated tests)
  • Critical code received unit tests by the developer
• Non-functional requirements
  • No sonar cloud issues