Issue 4870 revert individual fes msgs

[tomholub]

This PR reverts change of behavior introduced in issue #4348 without reverting all of the general code / refactoring changes. Various parts of the code can still handle sending more than one actual message per "send button" click, which may still come handy, with a note to refactor it away in year 2024 if it remains unused.

close #4870
close #4702 - I also fixed this as a byproduct. Since it's a single message sent to pubkey recipients and to password link recipients, as before, it does make sense again to attach the files to it (since we only have to upload them once, not once per recipient), which is what I've done.

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Tests (delete all except exactly one):

• Tests added or updated (WIP)

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[tomholub]

behavior looks good

[tomholub]

self review

[sosnovsky]

Looks good 👍