Allow Owners packages to be configured to ignore generated paths in Differential

Summary:
Depends on D19427. Ref T13130. See PHI251. Support configuring owners packages so they ignore generated paths.

This is still a little rough. A couple limitations:

• It's hard to figure out how to use this control if you don't know what it's for, but we don't currently have a "CheckboxesEditField". I may add that soon.
• The attribute ignore list doesn't apply to Diffusion, only Differential, which isn't obvious. I'll either try to make it work in Diffusion or note this somewhere.
• No documentation yet (which could mitigate the other two issues a bit).

But the actual behavior seems to work fine.

Test Plan:

• Set a package to ignore paths with the "generated" attribute. Saw the package stop matching generated paths in Differential.
• Removed the attribute from the ignore list.
• Tried to set invalid attributes, got sensible errors.
• Queried a package with Conduit, got the ignored attribute list.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19428