Add lots of test cases for parsing+serialization of full posts

[nylen]

We need to get a bunch of test cases in place for parsing and serialization. Preferably full posts' content that are tested as follows:

• Raw content
• List of blocks is verified against an expected value
• Optionally we can send some redux actions to simulate user changes
• Serialized to raw content again; ideally this content should all be exactly the same unless the user changed something, but it will take time to get there

Here is one example of a bug we would have caught with an approach like this: #626 (comment)

[nylen]

Closing; this is mostly done.