Restricted paste option for post editor

[Throne3d]

Right now we paste all sorts of styles, including foreground + background color, which often doesn't show up clearly to the end user. A couple of things that would be good to do here:

• Have a paste mode that only keeps "basic" formatting (eg bold, italic, underline, strike, presence of links - see Add a quote feature / better sanitize post HTML #23 (comment) for a similar idea)
• Warn when an element sets a foreground color without a background color (common when people copy from other locations and can mess up dark mode views)