Work on the same repository for the whole lesson

[astroDimitrios]

Morning branching everyone works on the same git-training-demo repo with write access.

Mention that permissions are now triage or just read (don't necessiraly have to change the permissions) before forking at lunchtime.

This means we have to get everyone's usernames before the lesson starts - add to instructor notes.
We can use a git-and-github team which asych people can request access to.

External people need to be added manually.