This is a squash of a bunch of commits releated to feature limiting. Includes making the upgrade path seamless by accessing billing through a WebView and making sure that the feature is enabled after upgrading, or canceled if the upgrade path is stopped by the user in any way. Included diffs --------------- Differential Revision: https://phab.nylas.com/D4078 Differential Revision: https://phab.nylas.com/D4136 Differential Revision: https://phab.nylas.com/D4137 Differential Revision: https://phab.nylas.com/D4143 Differential Revision: https://phab.nylas.com/D4147 Differential Revision: https://phab.nylas.com/D4171 Differential Revision: https://phab.nylas.com/D4173
