Copy & paste improvements

Several copy & paste improvements were added this week based on your helpful feedback, improving pasting content from both external applications and Manuscripts.io itself.

NOTE! Whilst the way copy & paste is handled in our new collaborative editing system is way more robust than the Manuscripts for Mac editor that precedes it, there are still known cases we don’t handle, and cases we are not aware of. Therefore, please do let us know of any unexpected cut / copy / paste behaviours via as Feedback topics on the community site !