Implement changes to MSC2285 (private read receipts)

[SimonBrandner]

Type: task
Implement changes to MSC2285 (hidden read receipts)
Requires matrix-org/matrix-js-sdk#2221
Requires matrix-org/synapse#12168

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This change is marked as an internal change (Task), so will not be included in the changelog.

Preview: https://pr7993--matrix-react-sdk.netlify.app
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[t3chguy]

LGTM otherwise

[codecov]

Codecov Report

Merging #7993 (774fbdc) into develop (3e31fdb) will decrease coverage by 0.07%.
The diff coverage is 30.00%.

❗ Current head 774fbdc differs from pull request most recent head a4c9ba7. Consider uploading reports for the commit a4c9ba7 to get more accurate results

@@             Coverage Diff             @@
##           develop    #7993      +/-   ##
===========================================
- Coverage    30.85%   30.77%   -0.08%     
===========================================
  Files          893      893              
  Lines        50793    50778      -15     
  Branches     12928    12926       -2     
===========================================
- Hits         15671    15628      -43     
- Misses       35122    35150      +28     

Impacted Files	Coverage Δ
src/components/structures/MessagePanel.tsx	66.02% <0.00%> (ø)
src/components/structures/TimelinePanel.tsx	1.66% <0.00%> (-0.01%)	⬇️
src/utils/Receipt.ts	0.00% <0.00%> (ø)
src/utils/read-receipts.ts	66.66% <60.00%> (-33.34%)	⬇️
src/components/views/rooms/SearchBar.tsx	0.00% <0.00%> (-89.29%)	⬇️
src/components/views/rooms/ReadReceiptGroup.tsx	8.33% <0.00%> (-21.67%)	⬇️
src/contexts/RoomContext.ts	100.00% <0.00%> (ø)
src/components/structures/RoomView.tsx	29.65% <0.00%> (ø)
src/components/views/right_panel/UserInfo.tsx	29.23% <0.00%> (ø)
... and 2 more