Allow user and admin to change Decred address

[xaur]

Afaik it is currently not easy to change the Decred address (I think it's called "voting address"?) because it requires directly editing the database.

Changing address is a legit use case if the user migrates to a new seed or just desires to rotate the address for privacy reasons.

Currently to change the address the user is advised to create a new VSP account, which is pita because it needs a new email address.

It would be nice if the user could change the DCR address himself to remove the load from the admin and also eliminate the roundtrip.

I guess admins might also need an ability to change the DCR address for the user in some circumstances.

Originally discussed in #426.

[UferePease]

After changing the ticket address, will there be any need to use the previous ticket address again? Should the old address be kept or discarded?

[xaur]

The old address need to be rememberd for two reasons. One is to allow the VSP to vote for the user until all tickets from that address are voted. Another is to present historical data on past tickets.

[raedah]

Recommended to close. This will be made redundant by planned privacy work.

[xaur]

@raedah generally agreed, question is how far away is the deployment? If it's a few months away I agree there's no need to spend effort on this. If it's e.g. 1 year away then this feature could make some use until then.