Concurrent edition should be tested

[benel]

In order to pave the way for students implementing #348, #250, #251, #345, a first acceptance test should be created for the current behavior which prevents overwriting the new version by someone else:

1. getting the new version of the block before getting in edit mode,
2. checking if the version has changed on the server on saving the new local version.

Please note, that, if I am right, for metadata and the first block, only the second behaviour is implemented yet.

Concurrent user acceptance testing is known to be hard, but this blog post has an interesting solution for this:
https://dev.to/cydavid/cypress-real-time-multi-user-testing-made-easy-1012