- Added new survey properties: `date`, `startTime`, and `endTime`.
 - Do not present survey if end time has passed [ID 49448805]
 - Do not present survey if current time is before survey's start time
 - Cases for surveys without start/end times
Fixing race condition between network status change and subscription notification.
 - A survey should not be presented to a user more frequently than the display limit (e.g. once per X)
 - [Fixes 49450933]
 - Added new `showOncePer` property to surveys (not yet being sent by server)
 - Last shown date saved in NSUserDefaults
Merge branch 'surveys' of https://github.com/apptentive/apptentive-ios into surveys
IOS-119 Implement short v. long survey text response type

Still need to have some clarity around whether short text response types should get a length limit from the server.
Merge branch 'surveys' of github.com:apptentive/apptentive-ios into surveys
 - No value for a survey's `show_once_per` is coming in as 0, not nil.
 - If it comes in as 0, now setting the ATSurvey's `showOncePer` property as nil
 - We could, instead, check later for 0 in addition to nil. But this seems easier and more inline with other survey properties.
Merge branch 'surveys' of https://github.com/apptentive/apptentive-ios into surveys
Refactoring and adding unit tests.

[#49450933 #49448805]
 - Saving an array of survey datesShown rather than the single last shown-at date
 - Still only checking the most recently added shown at date for 'shownTooRecently' test.
 - When 'show_once_per` is replaced on server by a limit + duration, we can do more interesting calculations.
Added survey `viewCount` and `viewPeriod` properties.
Removed `showOncePer` property
New `isEligibleToBeShown` convenience method for checking if a survey is within start/end times, within view limits, etc.
Currently not "spacing out" survey view limits at all… can be shown 3 times in the first minute of a month, then not until survey period has passed.
Updating English strings file.
Fixing incorrect parameter to error log.
Fetching surveys from the new survey index API using the conversation token.

Also submitting to the new survey API endpoint.

This involves renaming the old survey response records to be legacy survey responses, changing how the survey tasks are enqueued, and adding a new model version to CoreData along with a relevant ATSurveyResponse object.

[#53042301 #49449919]
Merge branch 'surveys' of github.com:apptentive/apptentive-ios into surveys
Refactoring a bit to make resetting the views be more explicit.
Fixing unit tests to work with Core Data.
Adding test of migration from v1 of data model to v2.
Added support for survey `end_time` in RFC-3339
[Fixes #53252033]