Add self-identification support

[mxbi]

No description provided.

[lewis-wf]

Our specifications for this are:

1. it needs to support anonymous self-identification (client side, nothing stored on the server). This is slightly inconvenient for the user, insofar as they need to re-select for each election, but I don't think that justifies the risks of trying to store in it a cookie or (god forbid) in our DB
2. users must be warned (big red box) that if this is a single position election (e.g. LGBTQ rep) then their vote might be public and therefore they could be outed by voting - this is why hidden voter list for self-identification elections #16 would be needed alongside
3. election admins should define what 'groups' are in the list for any given election