MM side of fix for various NPEs and loading issues in MegaMek/megameklab#1537.

Fix replaces a magic number in the BLK save code with a defined constant; the code was skipping Standard armor on SVs with Armored Chassis mod and leaving them in an un-loadable state since 0.49.19.

This also adds some handling for fields that had become mismatched between the old SV blk save code and the new loadSV() code, specifically, 'armor_tech[_rating]' which was using the long form to save and the short form to load, leading to undefined behavior.

Finally, there is an MML PR to disable the Armor Tech Rating chooser when non-BAR armor is chosen, to prevent confusion and also stop random fiddling with the chooser causing the Chassis Tech Rating (and thus weight) to fluctuate.

Testing:

• Loaded previously-failing files after adjusting them manually, then confirmed re-saving them with the patched code fixed all issues.
• Ran all three projects' unit test suites