Feature/variable snake length

[nbw]

Summary

Adds an adjustable _snake_startlength parameter, instead of the default length 3.

Code summary

• two migrations to add _snake_startlength to the games and game_form tables
• updated game specs to check some of the validations
• added snake start length input form
• default length is still 3, but can be adjusted to as low as 1

Screenshots

Background and impetus for PR

My team is a sponsor for battlesnake this year and we're hoping to have a bounty snake challenge that is like tron, where the tail never moves. I'm sure you know what that is, but to clarify dig this gem:

One way to accomplish that is to set the _snake_startlength to a really high number. The rest is just on us to make a decent snake.

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Just a personal comment, the code you wrote and the project is really sweet. Really easy to setup and follow. Worked exactly as expected.

[Dkendal]

Hey Nathan!

Thank you very much for this PR, this is actually a feature I've been meaning to tackle for a long time.

Really cool idea for a bounty snake, and thanks for your kind words!