Environment-dependent and conditional checks

• Improved the allgood DSL by adding optional conditionals on when individual checks are run
• Allow for environment-specific checks with only and except options (check "Test Check", only: [:development, :test])
• Allow for conditional checks with if and unless options, which can be procs or any other condition (check "Test Check", if: -> { condition })
• Added visual indication of skipped checks in the healthcheck page
• Improved developer experience by showing why checks were skipped (didn't meet conditions, environment-specific, etc.)
• New DSL changes are fully backward compatible with the previous version (new options are optional, and checks will run normally if they are not specified), so the new version won't break existing configurations
• Changed configuration loading to happen after Rails initialization so we fix the segfault that could occur when requiring gems in the allgood.rb configuration file before Rails was initialized