Resolve "Make CAS feature optional"

Closes #104

Description

See #104

Changes

• no more backend session id, no more mapping (session-id/cas-user) => replaced by user info directly stored in session
• no more mixing functionalities in backend signin api: split into: signin (via form), getuserinfo, logout
• remove CAS user save in DB => needs discussion
• remove jwt token, only use flask session for user info
• add cas variable for frontend environments
• update angular.json and package.json with new environment without cas
• update authentication unit tests
• allow user to delete his instances
• update README
• update CI (disabled e2e test) => to be changed later with correct docker images

As reminder for later:

• change db model (email may be better for primary key ? or using cas login with prefix ? )

How to test

Test both authentication methods