Rollback to LegacyContextFactory

There is an issue in `MediaLibraryPickerDataSource`, where its
`fetchController` keeps receiving updates and causes the UI to blink.

Author: crazytonyli

WordPress/Classes/Utility/ContextManager.m

@@ -50,7 +50,7 @@ - (instancetype)initWithModelName:(NSString *)modelName storeURL:(NSURL *)storeU
     self = [super init];
     if (self) {
         if (factory == nil) {
-            factory = [ContainerContextFactory class];
+            factory = [LegacyContextFactory class];
         }
 
         NSParameterAssert([modelName isEqualToString:ContextManagerModelNameCurrent] || [modelName hasPrefix:@"WordPress "]);

WordPress/WordPressTest/ContextManagerTests.swift

@@ -176,6 +176,7 @@ class ContextManagerTests: XCTestCase {
         // Discard the username change that's made above
         contextManager.mainContext.reset()
 
+        XCTExpectFailure("Known issue: the mainContext is saved along with the `ContextManager.save` functions")
         expect(try findFirstUser()?.username) == "First User"
     }