Add ability to swap server url on a davx5 account

[zamaudio]

Hi, I migrated my nextcloud from one domain to another, but I could not seem to find a way to rename the server url of an active davx5 account. This is problematic because I cannot migrate my active contact list without first doing a backup from my phone that can no longer sync, and delete the davx5 account and recreate it before restoring the additional contacts that were added between the migration.

[rfc2822]

Tracked in #571