Email templates

[felixrindt]

closes #372
related to #989

apperently building email html is terribly complex

• validate how embedded images get deliverd to actual mailboxes. Our provider marked versions with embedded svg as junk.
• build a "mail was read" mechanism (alternativly only "action was clicked") --> moved to do with Notification Framework Improvements #989
• clickable org name
• try out centering action buttons and header
• link to disable all email notifications
• Remove alt Text from top logo

Approach:

• Notifications provide a subject and body
• Plaintext is rendered just as html: with a template
• get_url got refactored to get_actions (label+url), which are rendered in plaintext and html. We could later use them as push notification actions.

Developing

I released the test plugin in a new version to include the view you see below.

[felixrindt]

[coveralls]

coverage: 88.896% (+0.1%) from 88.766% when pulling 7272ee3 on email-templates into 7cee58c on main.