iOS notifications are broken with Synapse 1.33.0

[manuroe]

Because of matrix-org/synapse#9919, the swift code and its strong type check crashes.

[manuroe]

We can probably run our integration tests against matrix-org/synapse#9919.

[manuroe]

This is probably the right time to start to:

• enable NS_ASSUME_NONNULL_BEGIN in our MXJSONModel classes and use nullable according to matrix.org spec
• split MXJSONModels.h into dedicated classes

We can start this with all JSON models related to the /sync response. It may worth to package all models into a dedicated subfolder.
The runtime crashes due to this synapse change should then be visible as build errors in our swift code.