This will allow us to say "a channel" instead of "this channel" when referring to a child thread in CreateSubThread.robotext, but it requires that we pipe through the threadID to whatever function is "rendering" the EntityText to a string.

Depends on D6516

I git grep robotextToRawString to make sure I am including threadID in all the places I should be, and tested CreateSubThread.robotext in a subsequent diff where it renders a ThreadEntity for a subchannel (not the current thread)