feat: Provide Email-module

[phorward]

Provides admin tooling for e-mail queues.

The module has to be sub-classed in a project where it is in use:

from viur.core.modules.email import Email


class Email(Email):
    pass

Currently requires these index.yaml entries:

- kind: viur-emails
  properties:
  - name: isSend
  - name: errorCount
    direction: desc

- kind: viur-emails
  properties:
  - name: isSend
  - name: creationDate
    direction: desc