Interpret whitespace after entity as the end of the entity
The easiest way to stop the user from inserting whitespace onto the end of an entity is to toggle the entity state of the whitespace that was just entered. This allows the user to continue drafting a message without editing the link content.
This is for pasted `<a>` tags that have been copied from a website. We probably also want to be storing entities for substrings of content that are determined to be URLs.