Updates to version-pinned vals

[stevekrouse]

If you version-pin a val (which happens by default) you won't get updates to that function. It'd be nice if we could have something kinda like dependabot to help folks upgrade.

Some ideas:

• Notification when a val you have pinned has a new version (debounced)
• Someone (us at Val Town) could review updates and approve them for them to go out to folks automatically
• Button that the publisher of a new version could press to "notify dependents of a new version", with a spot for a note

[nbbaier]

This is related to #67

[stevekrouse]

A dependabot like thing in userspace could be awesome