Extra empty block in read only mode.

[hsingh-ds]

Describe the bug
Editor is adding an extra paragraph block at end of the content. In read only mode there should only be the blocks that are in the "initialContent".

To Reproduce
Try to load editor data in read only mode. An extra block will get automatically added at the end.

[jaredly]

would be addressed by a fix for #631 I believe

[YousefED]

would be addressed by a fix for #631 I believe

This one is a bit different, because it's about editors with editable=false. I see two options combined with #631

trailingBlock: true (default) would not show a trailingBlock when editable: false I think this makes sense because I don't see a use for having a trailing block

Alternatively, we could allow trailingBlock: "auto" | true | false where auto does the above behavior and true / false always applies that boolean.

Edit: I think actually this should be solved in CSS for non-editable editors (hide the trailing block), because otherwise we'll trigger updates when switching the value of editable, which imo is not desired (especially combined with collaboration)