Allow several root comments

[frandavid100]

A single piece of text could spawn different conversations, and it would be nice if those conversations could happen separately.

To this end, a user should be able to highlight a piece of text and add his own root comment, thus starting a conversation thread, and a different user should be able to also highlight the same text and create a different comment, which would start a different thread.

[DanilaFe]

This seems at least partially relevant to #4. Multiple discussions could happen on multiple highlights over the same text, but selecting one of them is not possible right now.