Adds backend support for `is` operator with the `dm` operand. This
will deprecate the `is` operator with the `private` operand, but we
keep support for backwards-compatibility.
Note that there is some clean up of references to private messages
in the updated backend test. In commit 43ec7ed, the documentation
for `build_narrow_filter` wasn't updated for the rename of
`BuildNarrowFilterTest` to `NarrowLibraryTest`, so that's also
corrected in these changes.
The general API changelog and documentation updates will be done
in a final commit in the series of commits that adds support for
the various new direct message narrows.
ece752014c