Increase city strength

[HaneulCheong]

• Increased city strength
  • Pseudocloses the first part of General Balance Suggestions #31 (comment).
  • In short, Initial city strength is now on par with that of a Spearman.
    • Base city strength increased from 8 to 20.
    • City strength per pop increased from 0.4 to 0.5.
    • Bomb Shelter now gives 5 city strength instead of 2.
  • Made early-game rushes much harder while still allowing aggressive late games.
  • Also fixed a typo in Buildings.json.
• Hid Flood plains
  • Closes Floodplains #21.
  • Flood plains are now hidden from Civilopedia since they will not spawn.
• Fixed snow farm using vanilla tileset
  • Snow farm without hills now uses DeCiv's tileset.
  • Also fixed my mistake in 6a0906f:
    I forgot that multiple trailing spaces in markdown is a line break...

Related: #35