v17-pre8, v17-pre9

I think that this new update is in a pretty good place. Big thanks to everyone on Steam and especially in the Discord who has provided a lot of feedback on how to polish the new plane ordering mechanics. Hopefully this is a step in the right direction. If everything looks good, I will just do a few small tweaks and bugfixes and have the Puffin Planes Planes Update Part 1 out sometime in May.

v17-pre9
Changes:
- The "production line" concept has been partially split off into the "orders" concept.
- You can now only order the latest block of a given plane.
- Older blocks will be "retired" after all outstanding orders have been completed.
- Manufacturers will prioritize finishing outstanding orders for older blocks before moving on to newer ones.

- Removed the priority queue, replacing it with a new 'Priority Deal' mechanic. Priority Deals are first-come-first-serve and result in any existing orders being moved up in the queue rather than being put into a separate queue.

- Production lines now live on the Investment page.
- Production lines now have cities and randomly generated names.
- New blocks are now developed much more slowly.
- Retrofitting planes now costs more miles and service points.
- Lowered passenger spawning.
- Optimized passenger spawning.
- Balanced maintenance tiers; 2x maintenance now doesn't kick in until planes are 50 quarters old.
- Roebling speed now starts at 1900 kph up from 1250 kph.
Bugfixes:
- Fixed some bugs with the client panicking when an unowned plane is destroyed.
- AI now take into account how many planes they have already ordered when deciding whether or not to order more planes (derp).
- Fixed bug where AI would refuse to buy planes mid-late game.
- Fixed bug where locked airplane models would show on the block upgrade research page.
- Fixed bug where manufacturers might keep infinitely inflating the projected cost of a new production line and never actually build one.

v17-pre8
Features:
- Now receive a notification once per quarter about your old planes.
- Optimized passenger spawning. Passengers now spawn less frequently, and in larger batches. The overall behavior should probably be the same, just a bit coarser.
- Reworked Policies window. Removed aircraft policies and tried to merge some of them into airline policies.
- Bold orange text that lets you know that you will overwrite your other choice when switching between conflicting policies.
Changes:
- Unified dropdowns for sorting airplanes across network overview and flight assignment windows.