views: Reorder the update listing filters alphabetically

We're starting to have a lot of them, and others are still coming. This
should make the code slightly more readable.
Filter update listings by packages

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

This adds the required schema node, view filtering, and unit tests.
tests: Add a CVE to the test update
Filter update listings by CVEs

This adds the required schema node, 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? :)
Filter update listings by approval date

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

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

This adds the required schema node, view filtering, and unit tests.
Filter update listings by qa approval date

This adds the required schema node, view filtering, and unit tests.