Further improve copy and paste behavior

[max-nextcloud]

The fact that the markdown syntax is copied into the text/plain clipboard creates a confusing user experience. We received feedback from using Text/Collectives as a log and doing a lot of copy+paste from it into different other systems. Copying to the Signal Desktop client and to any mail client is literally broken the way it works now. And while #5487 improves the situation a bit (when the copied content stays in one paragraph or block node), the annoyance stays when copying larger parts of a document.

Markdown syntax that is confusing

• Copy a list of email address links that spans over several blocks (e.g. [[email protected]](mailto:[email protected])\n\n[[email protected]](mailto:[email protected]) into the recipient field of any mail client (Thunderbird, Roundcube)
• Copy two paragraphs of text with bold, italic, etc formatting into the Signal Desktop Client or the text-only body field of your mail client

Originally posted by @mejo- in #5487 (comment)

Markdown syntax that might be helpful

source	plaintext (Nextcloud 27)	markdown (Nextcloud 28)

_Originally posted by @max-nextcloud in #5487 (comment)

[max-nextcloud]

For reference - here's the output of LibreOffice Writer when copying and pasting the above example:

Shopping list:
    • icecream
    • vegetables
    • fruits
tables
are
hard
to
get
right
let's
go
shopping
Let's bake some cake:
    1. get the ingredience
        1. go to the super market
        2. buy
    2. mix them
    3. bake them
    4. enjoy

It also turns mailto links into plain text, ignores formatting such as bold or italic and links.