Add section preview mode to API action=parse

In https://gerrit.wikimedia.org/r/#/c/67266/, it was enabled automatically
when a section was specified in preview mode. However, this only works for
previews of a saved page's content, failing for user input previews which
might contain several sections of various degrees of nestedness.

Change-Id: I987144e5e315ed9e0a67f599083461a78879c0df
Merge "Add section preview mode to API action=parse"