New control strings are not displayed correctly in warning about unbound keys

[wookieejedi]

Under some circumstances, FSO will warn the player if a key is unbound if that they need to press to complete a training event. This logic though only works for keys with XSTRs from 507-619. Any control that was added with a different XSTR will not be picked up correctly, and thus the message displayed to the player will be incorrect, confusing, and not helpful.

Here is an example, where the incorrect XSTR look resulted in a very confusing message:

To fix this missiontraining.cpp around line 757 needs to account for the newer controls as well. I'm going to work on a PR for this.