Accessibility: Fix Block Appender Accessibility

[youknowriad]

closes #4829

This PR adds a label and a role to the appender to make it discoverable by text readers.

[afercia]

@youknowriad thanks.
Sorry for the bad example label used on the screenshot on the issue (Add block (appender)), that was just for testing purposes. I've changed it to Add block and updated the snapshots to make the tests pass.

Maybe we should find a better way to distinguish the controls that insert a new block between existing block (Insert block) from the ones that add a block at the end of the post (Add block), but we can probably iterate later.

[youknowriad]

Thanks for the updates @afercia