Improve join game process

Change HTTP method appropriately to POST (incorrectly was GET)
Add in pop-up modal for verification that user wants to join
Move some js files that were games#show only to entire app

Lay some basic groundwork for letting creator accept/reject someone &
letting the two players arrange the game settings
Remove unnecessary, non-idiomatic usage of 'self'
Fix bug in gameplay feature specs (integration tests)

Do some small misc code clean ups/improvements
Implement resign game feature

Add some minor improvements to join game modal