JX.Mask: Make fixed position, sigilized, and classable
JX.Mask: Make fixed position, sigilized, and classable

Summary:
  - Make JX.Mask rely on "position: fixed" instead of "position: absolute" plus JX.$V plus document resize events. This fixes masks which do not resize properly when the window is resized.
  - Make JX.Mask have a "jx-mask" sigil on the element. This allows callers to listen for mask clicks to, e.g., close a lightbox.
  - Make JX.Mask take an optional class, so you can have multiple different mask styles.

NOTE: This breaks masks in any browser which does not support "position: fixed;", and requires mask rules be updated from "position: absolute" to "position: fixed". I assume Facebook isn't using JX.Mask so this isn't actually an issue, but let me know if you need more of a migration path.

Test Plan: Will use in next diff.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: paulshen, aran

Differential Revision: https://secure.phabricator.com/D3797

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