Update Buildscripts with new SpongeGradle

[gabizou]

SpongeGradle | SpongeAPI | SpongeCommon | SpongeVanilla | SpongeForge

This is a working PR with the development lock step with SpongeGradle. While this technically is compatible with Gradle 4.10.3, I've elected to use the latest Gradle 5.6.2 (as of current writing) to take advantage of the improved Kotlin-DSL since those versions, and configuration avoidance throughout the build process.

Some changes:

• Constants are just constants in files
• Things that should've been made properties are now properties and/or functions in SpongeGradle or in the gradle.properties of their respective projects
• Updated to use appropriate Gradle ideologies with configuration avoidance (lazy evaluation)
• Reduced the overall build file size and using as much handling with the gradle plugins to configure most things we've otherwise duplicated and/or scripted with other scripts.