Post Title: Insert initial block as provisional

[aduth]

This pull request seeks to update the behavior of pressing enter from the title field to leverage the same insertDefaultBlock as the default block appender and betweenserter for consistency and so that it inherits the same "provisional" block behavior introduced in #5417.

Testing instructions:

1. Create a new post
2. Insert a title
3. Press enter
4. Press arrow up
5. Note that after focus transitions into the title, the block which had been inserted after step 3 has been removed (the "Write your story" prompt returns)