Add game menus, among some other bug fixes

[nucleartide]

Changelog

What is the "shippable increment" of this pull request?

• Add initial start menu to game
• Many bug fixes for player grid-aligned movement
• Add MathHelpers::NegativeFriendlyFmod
• Add StartMenuWidget
• Simplify PR template

Dev Checklist

• First draft. Implement a working first pass at the feature/problem.
• Final draft. Perform a general review of Files changed and revise anything that needs to be cleaned up.
• Debuggability. Ensure that all features within this pull request have debug views. - Good enough. Expose more properties via UPROPERTY(VisibleAnywhere), and draw debug geometry in the future if needed.
• Playtest. Playtest through game to ensure everything works.

Release Checklist

• Cut a new build in Unreal, and upload a GitHub release.
• Consider discussion points to write about in readme.