add git collaborating task

[cifarquhar]

Adds a new backlog task for trainees to practise branching, creating PRs and merging outside the usual CYF workflow.

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[illicitonion]

LGTM - let's remove the HTML+CSS LO - I would be perfectly happy if someone replaced this exercise with collaboratively writing a poem or something, and having HTML+CSS as a LO suggests that wouldn't be ok