Previously `NarrowTerm` is only used in our event-handling paths and to
a lesser extent in the `detect_narrowed_window` in `view/home.py`. Both
of which haven't yet support or handle the `negated` term.
Since we're planning to parse a narrow URL into narrow terms (like in
`hash_util.ts`) in the web app, we're going to need a `NarrowTerm`
dataclass with all three flags.
This commit adds the `negated` term to `NarrowTerm` and adds a
`NeverNegatedNarrowTerm` which is a subclass of `NarrowTerm` that always
has the `negated` flag as `False`, so functionally it is the same as the
current `NarrowTerm`.
c861bdd494