[Feature] Add cruise speed Mach/TAS modes switch

[gzhynko]

Add a UI switch (button, select etc.) near the Cruise Speed field to let the user decide what type of cruise speed their briefing should specify. This would also require a new column in the Briefings table.

Edit: Actually, on a second thought, the new column can be avoided. The speed value can be checked on the View Briefing page and, if it’s higher than 4, display it in TAS rather than in Mach.