[Software] Let people sign up for shifts

[moeffju]

For each session at an event, let people sign up for specific languages, positions(?) and/or times(?). Would be nice if people could add tags or comments such as "I have translated this speaker before" or "I'm a subject matter expert" or "I am comfortable interpreting weird-ass art performances" etc.

Need to think about: Do we want any sort of workflow, any sort of rights system etc. like "shift managers" to confirm people's assignments? At C3 and re:publica we just talked about everything in the group which worked well, so maybe comment threads on each of those? but maybe not in the same system but in our Rocket/Zulip/whatever?