Publish Scala.js 1.0.0-M7 artifacts

[ashawley]

When I made #473, I didn't consider folks might want 1.0.0-M7 artifacts for Scalacheck 1.14.0 for Scala 2.11 and 2.12 (in addition to 2.13.0-RC2). These versions are likely useful for downstream libraries who cross-publish multiple binary versions of Scala.js.

MiMa is enabled in the 1.14.0 branch, but the way it is configured is likely a hindrance to publishing Scala.js. There may a way to improve it.

FWIW, I predict 1.0.0-M7 will be stable, so the 2.11 and 2.12 builds won't have to be released when 2.13.0 final is released. Only 2.13.0 artifacts will be needed.

[ashawley]

You'll probably be publishing 1.0.0-M8 artifacts while publishing 2.13.0-RC3 artifacts for #478.