The unsubscribe landing page is not shown

My bad. Copied over the test steps from #8395.

Will add a confirmation toast. Good idea!

Tested, worked fine, cool stuff!

Two nitpicks and a bug:

1. Location of the unsubscribe button. The current location crowds the lefthand side and doesn't really fit with the styling of the current header. I would prefer something like this:
   
   cc @jancborchardt

Alternatively add a few pixels of margin to the button:

Before	After

2. The toast for the unsubscribe stays visible for a long time compared to other toasts.
3. the button is visible on mobile but messes up the header:

I'd hide it for now and fix it at a later date.

Location of the unsubscribe button. The current location crowds the lefthand side and doesn't really fit with the styling of the current header. I would prefer something like this:

See #5387 and #5387 (comment) (bottom). The button was placed there intentionally.

The toast for the unsubscribe stays visible for a long time compared to other toasts.

It uses the default for success toasts: https://github.com/nextcloud/nextcloud-dialogs/blob/c16397fe19f43a4def95ba1dcab13ebf8929232c/lib/toast.ts#L45-L48

the button is visible on mobile but messes up the header

Tracked in #8191

Cool 👍

A mail receiver can do a one-click unsubscription by performing an
HTTPS POST to the HTTPS URI in the List-Unsubscribe header. It sends
the key/value pair in the List-Unsubscribe-Post header as the request
body.

The POST content SHOULD be sent as 'multipart/form-data' [RFC7578] or
MAY be sent as 'application/x-www-form-urlencoded'. These encodings
are the ones used by web browsers when sending forms. The target of
the POST action is the same as the one in the GET action for a manual
unsubscription, so this is intended to allow the same server code to
handle both

To comply with the RFC, we need to send the List-Unsubscribe-Post key/value pair to the unsubscribe url.

Thanks @kesselb. Didn't see any header values in the messages I tested with.

To add more fun:

As described below, the
message MUST have a valid DomainKeys Identified Mail (DKIM) signature
that covers at least the List-Unsubscribe and List-Unsubscribe-Post
headers.

The message needs at least one valid authentication identifier. In
this version of the specification, the only supported identifier type
is DKIM [RFC6376]. Hence, senders MUST apply at least one valid DKIM
signature to the message.

The List-Unsubscribe and List-Unsubscribe-Post headers MUST be
covered by the signature and included in the "h=" tag of a valid
DKIM-Signature header field.

If the message does not have the required DKIM signature, the mail
receiver SHOULD NOT offer a one-click unsubscribe for that message.

Possible library to validate DKIM: https://github.com/pimlie/php-dkim (horde already has a couple of methods to fetch and normalize headers, maybe we don't need an additional lib).

Plugin for Roundcube: https://github.com/pimlie/authres_status (very helpful readme with lots of information).

@kesselb does your rebase mean this is done and ready to review? :)

@ChristophWurst ready to review.