Various changes to the Article class

[Eitot]

• Replaces -markRead:, -markDeleted: etc. methods in the Article class with read–write properties
  This simplifies the interface of the class. I could not find a reason why the -markX: methods were used instead of making the corresponding properties writable.
• Changes the type of status property of the Article class from NSInteger to ArticleStatus
  This provides better compatibility with Swift.
• Uses automatically synthesised ivars and accessors in the Article class
  This gets rid of needless boilerplate code.
• Extends the NS_ASSUME_NONNULL_* annotation block to cover the whole Article class
• Declares the designated initialiser of the Article class and marks the inherited -init method as unavailable
• Uses ivars instead of the dictionary for certain properties in the Article class
  These properties are set in the initialiser, whereas other properties may not be set at all.
• Expands article unit tests to test initial values of the Article class
• Removes duplicate test methods from the VNAArticleTest class
• Renames VNAArticleTests.m to ArticleTests.m