Add a user-interface for changing user settings

[BjarniRunar]

There needs to be a simple user interface that lets the user change their name, e-mail address and password at the very least.

A slightly more advanced interface should allow users to specify the following:

• Whether they'd like to be signed up to various mailing lists
• Their public PGP key

How to do it:

• E-mail and password changes require calculating a new JSON storage file name and using the user-mv.py script to rename the user's data file. A new method should be defined in hoipoi.js for performing such rename operations.
• Other changes are a matter of defining the variable names in the user JSON, and using hoipoi.user_set() method to set/update the values.

[bnvk]

@BjarniRunar is the reason for the public PGP key simply, because PGP ENCRYPTION FTW? 😁 or do you have other plans...?

[BjarniRunar]

In particular, it would be nice if password-reset style messages from Hoi Poi itself were not sent in the clear.

Mailing list posts are not really secrets, so it might not be necessary to encrypt those from our point of view - but recipients might still prefer they be encrypted for reasons we cannot (and do not need to) know.

If we think of Hoi Poi as a general purpose election tool, security is actually rather important in some cases. The way we are using it is not easily abused, so Mailpile's requirements are not very stringent, but it'd be cool if the tool matured over time into something suitable for Serious Elections. You never know. :-)