What does this MR do?

This is an alternative to !902 (closed), and it may be simpler.

first of all, fetch from the buffer all the notes we may need (including replied-to ones)
then, distinguish between "we know there's no myReaction" and "we don't know if there's any myReaction"
finally, look at both a note and the note it's replying to

Tested: I see correctly-marked reactions in:

a lone note
a note replying to a note
the reply of 2
a renote of 1
a renote of 2
the reply of 2 when shown via 5

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