R1:49af4165bc70

Support rendering arbitrary sections in the middle of a Harbormaster build log so links to line 3500 work

Summary:
Depends on D19162. Ref T13088. When a user links to `$1234`, we need to render a default view of the log with a piece at the head, a piece at the end, and a piece in the middle.

We also need to figure out the offset for line 1234, or multiple offsets for "1234-2345".

Since the logic views/reads mostly anticipated this it isn't too much of a mess, although there are a couple of bugs this…