magit-insert-{rebase-sequence,status-rebase-lines}: move definitions

This is the beginning of the end of the top-level section "Sections".
Now that the accidental differences between functions that implement
Magit sections have been removed, there no longer is a good reason
to keep them in one place.
magit-insert-status-merge-line: move definition
move definitions of bisect functions below those of bisect commands
rename magit-{bisecting => bisect-in-progress}-p

Do so for consistency with similar predicates
like `magit-rebase-in-progress-p'.
make "Status Sections" a subsection of "Modes"

Rename the top-level section "Sections" to "Status Sections" and turn
it into a subsection of top-level section "Modes" and into a sibling
of "Status Mode".