Allow Legalpad document managers to add signature exemptions

Summary:
Ref T5532. Allow document managers to add exemptions, which act like signatures but are tracked a little differently.

The primary use case for us is users who sign a corporate CLA and need a user-level exemption if they don't want to sign an individual CLA.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5532

Differential Revision: https://secure.phabricator.com/D9795

resources/sql/autopatches/20140701.legalexemption.1.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_legalpad.legalpad_documentsignature
+  ADD isExemption BOOL NOT NULL DEFAULT 0 AFTER verified;

resources/sql/autopatches/20140701.legalexemption.2.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_legalpad.legalpad_documentsignature
+  ADD exemptionPHID VARCHAR(64) COLLATE utf8_bin AFTER isExemption;