This commit adds proper error messages when mail sending via the
mail queue has failed. It also refactors the MailTemplateService that it
does not use duplicate code to send mails.

[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

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