Modernized FBSDKShareKit

Summary:
This diff modernizes the `FBSDKShareKit` to support modern Objective-C syntax to increase type safety with both ObjC and Swift.

**General Changes**

* Removed Deprecated Methods, Classes, Properties
* Typedefs for all Objective-C blocks
* Used `instancetype` for inits
* Updated Xcode Projects/Schemes via. "Validate Project" option in Xcode
* Made `init`/`new` unavailable where/when possible
* Confirm `NSError **` throws on Swift
* Used `NS_SWIFT_UNAVAILABLE` where makes sense
* Ensured `NS_DESIGNATED_INITIALIZER` is used
* Added `NS_TYPED_EXTENSIBLE_ENUM` where made sense

**Updated Properties**

* Changed getter methods to `readonly` properties
* Changed getter/setter methods to `readwrite` properties
* Changed singletons to class properties
* Used dot notation for access to properties
* Added `getter=` for certain properties (e.g. `BOOL`)

**Added Nullability Annotations**

* Used `NS_ASSUME_NONNULL_BEGIN` & `NS_ASSUME_NONNULL_END` for files
* Used `_Nullable` for typedef blocks
* Used `nullable` & `null_resettable`
* Collections/Dictionaries `nonnull` when at all possible

**Added Generics**

* `NSArray` & `NSMutableArray`
* `NSSet` & `NSMutableSet`
* `NSDictionary` & `NSMutableDictionary`

**Added `NS_SWIFT_NAME`**

* Removed prefixes in favor of Swift namespaces
  * `FB` prefix for UI elements
* Added Dot notation where makes sense
* Swift init for class method creation methods

Differential Revision: D9843539

fbshipit-source-id: d15f5ccadbad1b3b07b482f068e799a112ec52d2

FBSDKShareKit.podspec

@@ -86,7 +86,6 @@ Pod::Spec.new do |s|
                         'FBSDKShareKit/FBSDKShareKit/FBSDKShareMessengerURLActionButton.h',
                         'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKShareDefines.h',
                         'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKShareError.{h,m}',
-                        'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKShareLinkContent+Internal.h',
                         'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKShareOpenGraphValueContainer+Internal.h',
                         'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKShareUtility.{h,m}',
                         'FBSDKShareKit/FBSDKShareKit/Internal/FBSDKVideoUploader.{h,m}'