Beginning a new TestContextManager to assist better unit testing

No incremental upgrade changes yet
Readded CoreDataConcurrencyTest to the test target
Notifications: Prevents SectionFooter from rendering
NoteTableHeaderView: Updates Style
NoteTableHeaderView: Fixes incorrect height assignment
NoteTableHeaderView: Implements custom separator
NoteTableHeaderView: Updates Style. Part II
NoteTableHeaderView: Updates Style. Part III
Make saveContext a class method rather than an instance method
Switched back to instance save methods on ContextManager

Also removing CoreDataTestHelper and it's horrible swizzled methods.
Fixed problem with shared instance being nil unexpectedly

Disabled two unit tests because they misbehave with Notifications being fired off
Merge pull request #2575 from wordpress-mobile/issue/2502-fixes-unwanted-horizontal-lines-in-notifications

Issue/2502 fixes unwanted horizontal lines in notifications
Added ALIterativeMigrator and unit tests for 19 -> 21 migrations
Iteratively migrate a store to prevent missed mappings
Updating Lookback.io

Updating to latest version which fixes some bugs with iPhone 6/6+.
Updating Lookback.io for reals

Last commit got eaten up by a bad rebase.
Using Mixpanel for feature flagging of Helpshift
Removing Taplytics.
Updates Crashlytics
Updates Simperium to Mark 0.7.1