Filter update listings by CVEs

This adds the required schema node, validator, view filtering, and unit
tests.
Filter update listings by locked state

This adds the required schema node, view filtering, and unit tests.
models: Default Update.pushed to False

Turns out that a default is necessary if we want to run queries like
this:

    db.query(Update).filter_by(pushed=False)

The above will not return the right updates, because they will default
to None rather than False.
Filter update listings by pushed state

This adds the required schema node, view filtering, and unit tests.
Filter update listings by qa_approved state

This adds the required schema node, view filtering, and unit tests.
Filter update listings by releng_approved state

This adds the required schema node, view filtering, and unit tests.
Filter update listings by security_approved state

This adds the required schema node, view filtering, and unit tests.
models: Default Update.severity to unspecified

Turns out that a default is necessary if we want to run queries like
this:

    db.query(Update).filter_by(severity=UpdateSeverity.unspecified)

The above will not return the right updates, because they will default
to None rather than unspecified.
Filter update listings by severity

This adds the required schema node, view filtering, and unit tests.
models: Default Update.suggest to unspecified

Turns out that a default is necessary if we want to run queries like
this:

    db.query(Update).filter_by(suggest=UpdateSuggestion.unspecified)

The above will not return the right updates, because they will default
to None rather than unspecified.
Filter update listings by suggestion

This adds the required schema node, view filtering, and unit tests.
views: Remove old comment

That's probably flexible enough now? :)