Electorates updating on server via 'my MPs' discovery

[vteague]

Suppose a user does the following, in order:

• Starts to make an account

• skips the 'find my MPs' step

• completes their account creation
  ... (possibly some time later)

• clicks the 'my MPs' button

• gets directed to the 'find my MPs' step

• completes the 'find my MPs' step, i.e. finds some MPs.

We do not currently have an option for them to save their newly-found electorates on the server. In particular, if they go to their account page, the (new) electorates will be there, but it won't realise they're new (i.e. have never been updated on the server).