Add end-to-end (Cypress) tests

[gturri]

Here is a list of some regression we almost had, and hence for which a safety net would be appreciated:

• user can login (nb: can break if the Symfony ad hoc routing middleware does not allow-list that endpoint anymore)
• user can logout (nb: can break if the Symfony ad hoc routing middleware does not allow-list that endpoint anymore)
• once user is logged out, the login screen is displayed (nb: this almost ended up broken when refactoring from ad hoc routing to angular router)
• Slack outdated members list is not reloaded when user just goes from one tab to the other (nb: there could be a regression on it for instance if someone tries manipulating the Observable directly instead of the wrapping Promises in the DataProviderService)
• the member table can still be sorted