Read More Block: Don't allow multiple

[thehenrybyrd]

Expected behavior

After one More Block is added, no more should be possible.
In Calypso, only one More Block can be added. When one already exists, the More Block is greyed out and can't be added again.

Actual behavior

It's possible to add additional More Blocks.

Steps to reproduce the behavior

1. Create a new post
2. Add a More Block
3. Add another More Block

Tested on iPhone SE, (iOS 12.2) running WPiOS 12.7

Hat tip @rachelmcr