Add colour to AI private channel (144.7)

Changes the AI private channel colour to magenta (#ff00ff), and changes
the marker from [144.7] to [AI Private].

Magenta is far enough from the Science channel (#993399) to be obvious,
but I'm open to alternative suggestions.
Merge pull request #4648 from GinjaNinja32/master

Add colour to AI private channel (144.7)