Migrate sbt-builds-for-ide into uber-build

This migrates sbt nightly build features into the uber-builds repo directly.

* Can build sbt nightly releases
* Can directly modify branches used to build sbt itself, to test upcoming features against the IDE
* sbt version is loaded directly from sbt build configuration.

Not implemented:

* Cut sbt releases with this script / add configuration to do so.
* Nightlies published to the correct repository

Review by @skyluc @adriaanm

Notes for review:

1. What documentation should be written/updated (I'll fix before merge).
2. Any additional features required?
3. Will this interfere with IDE's own releasing?
4. Are we fitting the style/spirit of the script?