Highlight @mention when rendering an annotation in the client

[acelaya]

Annotations that contain mentions will include the user key prefixed with the @ character.

They should also have a mentions array with the user information. Before rendering the annotation text, we should replace the key-mention with the corresponding username-mention` or display-name-mention.

@mentions should also be links, highlighted in a different color, that take you to the user profile.

Proper accessibility should also be taken into consideration, like making sure the link is underline to not trust just in color contrast.