This setting is more flexible than the existing REMAILER_LIMIT_TO_USERS config
option. The config option is therefore deprecated and will be removed in the
next major version.

Previously Unix UIDs/GIDs were allocated by using the highest used ID + 1.
This caused ID reuse when the newest user/group was deleted. In addition, the
implementation did not work on MariaDB (at all, it was not possible to create
users/groups).

The new approach accounts for all IDs ever used regardless of whether or not
users/groups are deleted. It always allocates the lowest ID in the configured
range that was never used.

Aside from the different allocation algorithm, this change introduces a
generic locking mechanism and prerequisites for testing migration scripts.

Enforces uniqueness of (verified) email addresses across all users. Email
addresses are compared case-insensitivly and Unicode-normalized. The new
unique constraints are disabled by default and can be enabled with a CLI
command. They are planned to become mandatory in uffd v3.

A lot of software does not allow multiple users to share the same email
address. This change prevents problems with such software.

To enable this feature run the command:

  uffd-admin unique-email-addresses enable

The commands reports any issues (e.g. existing duplicate addresses) that
prevent enabling the feature.

This change also introduces a generic mechanism to store feature flags in the
database and improves error handling for login name constraint violations.

Restructure tests into views/models/commands subdirectories to mirror the new
source tree structure introduced with ac731bf4 (Restructure source tree).