Link2Mail: email templates for mail action

[sleidig]

As a user I want to use prepared email templates to be able to communicate faster and more consistently.

Extending the very basic functionality of #3159, users should have the option to prepare templates with fixed subject and body text that can be selected with the "Send E-Mail" action. The user should see a selection dialog, similar to the popup shown for "Generate File". A dropdown offers all available templates to choose from. After selecting one and confirming the button "Open in email client", the local email app (e.g. Outlook) opens prefilled with the TO address as well as subject and body text from the template.

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Suggested implementation steps:

1. Extend the "sendMail" method of the service, called by the existing entity action, to include a subject and body text (e.g. see https://stackoverflow.com/a/4782084/1473411) (maybe start with a static text for testing)
2. Implement an Entity Type (isInternalEntity=true) for the Templates
   1. a similar architecture to the template-export module can probably be followed. It will also be best to keep all files in a feature folder similarly
   2. create and register views to list and edit (details) relevant fields of the template entity
   3. add a menu entry in the admin overview to this list view
3. Implement a simple dialog component to select available templates (and offer a link to create new templates) similar to the one for generating files.