Avoid looking up settings during timeline rendering

[robintown]

In a particular room with lots of hidden events that I tested this against, this sped up timeline renders on back-pagination from 1698 ms to 108 ms.

The original plan in element-hq/element-web#21740 was to then write a test to ensure that no settings get looked up during timeline rendering, but there's quite a lot of them still. For posterity, here's the output of console.log(settingsSpy.mock.calls) in that test:

[
    [ 'feature_jump_to_date' ],
    [ 'UIFeature.timelineEnableRelativeDates' ],
    [ 'feature_thread' ],
    [ 'feature_msc3531_hide_messages_pending_moderation' ],
    [ 'feature_use_only_current_profiles' ],
    [ 'feature_use_only_current_profiles' ],
    [ 'feature_use_only_current_profiles' ],
    [ 'feature_mjolnir' ],
    [ 'TextualBody.enableBigEmoji' ],
    [ 'feature_thread' ],
    [ 'feature_thread' ],
    [ 'showCodeLineNumbers' ],
    [ 'Pill.shouldShowPillAvatar' ],
    [ 'Pill.shouldShowPillAvatar' ],
    [ 'feature_thread' ]
]

Type: defect

Closes element-hq/element-web#21740.

---

Here's what your changelog entry will look like:

🐛 Bug Fixes

• Avoid looking up settings during timeline rendering (#8313). Fixes Rooms with lots of hidden events render very slowly element-hq/element-web#21740.

Preview: https://pr8313--matrix-react-sdk.netlify.app
⚠️ Do you trust the author of this PR? Maybe this build will steal your keys or give you malware. Exercise caution. Use test accounts.

[codecov]

Codecov Report

Merging #8313 (765800c) into develop (f97572f) will increase coverage by 0.03%.
The diff coverage is 70.58%.

@@             Coverage Diff             @@
##           develop    #8313      +/-   ##
===========================================
+ Coverage    29.98%   30.02%   +0.03%     
===========================================
  Files          879      879              
  Lines        50087    50093       +6     
  Branches     12747    12748       +1     
===========================================
+ Hits         15017    15038      +21     
+ Misses       35070    35055      -15     

Impacted Files	Coverage Δ
src/Unread.ts	29.41% <0.00%> (ø)
src/components/structures/RoomView.tsx	29.65% <0.00%> (ø)
src/components/structures/ThreadPanel.tsx	23.07% <ø> (ø)
src/components/structures/TimelinePanel.tsx	1.66% <0.00%> (ø)
src/components/views/rooms/ReplyTile.tsx	1.92% <0.00%> (ø)
src/contexts/RoomContext.ts	100.00% <ø> (ø)
src/settings/Settings.tsx	63.15% <ø> (ø)
src/utils/exportUtils/HtmlExport.tsx	6.32% <0.00%> (ø)
src/utils/exportUtils/JSONExport.ts	0.00% <0.00%> (ø)
src/utils/exportUtils/PlainTextExport.ts	27.63% <0.00%> (ø)
... and 9 more

[turt2live]

argh, didn't mean to clear review.

[turt2live]

otherwise lgtm - thanks for taking a look at this!

[turt2live]

(merged develop to try and fix angry CI)