[TASK] Add allmarker as default template content, fixed a bug where the template name could not be resolved

[TASK] Fixing some code issues, adding new ts configuration for path to automatic registration files

[FEATURE] Add attachments to mail queue

Closes #3

See merge request !9

Enable the mail queue to store attachments and send them later

We use a PlaintextService to convert the HTML body to a plaintext
version of it to be able to send it alongside the mail

[BUGFIX] Reset template for current domain only

See merge request !8

If a mail is sent via the mail queue, we must also set the sending_time
to prevent the scheduler command controller from sending the same mail
that has been send via the mail queue again.