Improve consistency of single line output from RTE

[artcodespace]

When creating a message from scratch, typing the message "sample text" and sending the message will send a message with content sample text. However, if you insert a linebreak (shift + enter on mac, dependent on settings), then press backspace (to return to a single line) and then type a message "sample text", the message content will be <p>sample text</p>.

This will cause issues with commands (if a user has done anything spanning multiple lines then used deletion to return to a single line) as they depend on checking the beginning of the message to see if it contains special text.

Proposed solution:

• When a user makes a change that moves from multiple paragraphs to a single paragraph, we should move the contents of that single paragraph to the root node and then remove the paragraph tag

[artcodespace]

Relates to #704