App:a legacy discussion that doesn't allow threading

Build 6785 on cemms1/address-darkmode-incompatibility-in-discussion
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Baseline History
Build 678552a0184
Current build. There are unaccepted changes, so build 6687 is still the baseline.
Build 63919ce985a
Accepted by Anna Beddow.
Build 5728fd4f18e
Accepted by Sophie.
Build 5634851d814
Accepted by Sophie.
Commenting is disabled because there's a newer build on this branch