Allow alternate, public account name

[esnickell]

Use case (mine, in this case):

1. User chooses padherder account name matching their email, intending padherder account to stay private
2. Months later, user wants to show padherder account in public forum, but does not want to leak account name, which others could guess are tied to a gmail account.

Options are to start over with padherder, and re-enter all the data, or...

padherder allows account to pick alternate account name used for public url.

[ngamber]

If this feature comes to fruition, this would also be an option:

https://github.com/madcowfred/padherder-issues/issues/455