iMip email should include images as attachments rather than HTML references to URLs on server

When an invitee email is associated with a calendar invitation, the invitee receives iMip emails as the invitation is made, updated, and canceled.

Unlike most other emails sent by the nextcloud server, the iMip emails do not necessarily go to nextcloud registered users - they can go to anyone. Furthermore, if the nextcloud server is hidden within an organization behind a firewall, these emails can go outside the organization, to users who do not even have access to the server.

We support the ability to suppress accept/decline buttons/links when a nextcloud server is not on the open internet (see issue #12156). However the iMip emails still contain links to images, such as the logo and various embedded graphics decorating the location, time, description of the event.

I propose that these images, which are small, be attached to the iMip email and referenced in HTML as "cid:..." URI's. This has two useful effects:

1. Users who do not have access to the server will see properly formatted HTML emails, complete with images.

2. For users who do have access to the server, URL images may be construed as privacy-breaching beacons, putting entries in the server logs that show when and where messages were read.

The downside is that iMip emails will be somewhat larger, as the images are attached to them.