Allow saving multiple images in a conversation

[FeuRenard]

I have:

• searched open and closed issues for duplicates
• read https://github.com/WhisperSystems/Signal-Android/wiki/Submitting-useful-bug-reports

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Feature Request

There was #5088 asking for multi-selecting images in "All images" and then saving them. It was closed without actually having multi-select & save implemented.
I am asking for multi-select images in a conversation and then save them.

Steps to reproduce

• go to a conversation
• long press a picture message
• tap a second/third/... picture message

Actual result: No save button is shown.
Expected result: A save button should be shown and allow to save the selected images.

[nysander]

when this feature will be deployed?

[FeuRenard]

Please don't bump issues! To vote for an issue, feature or PR you can add a positive reaction (e.g. 👍) to the initial comment.

[automated-signal]

GitHub Issue Cleanup:
See #7598 for more information.