Active private receipt support

[greentore]

Description

Adds an option to send private read receipts if one's homeserver supports them.
Marked as a draft because the toggle is awkwardly placed in security options.

Fixes #240 when coupled with #1108

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

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[github-actions]

Preview: https://1109--pr-cinny.netlify.app
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[ajbura]

This looks fine. But I am still not sure of if we want people to hide their read receipt but still able to see what others are reading. I believe it will change persons behaviour of being a part of community to being a spy like in community.

[VinceUB]

This type exists in matrix-js-sdk/lib/@types/read_receipts, why not just use that?