Update many Build.scala references

Removed as many references to `Build.scala` as I could find, and updated
them and the settings they were suggesting to update to `build.sbt`
format.

Fixes #1987
Update devError.scala.html
Update devNotFound.scala.html
Fix typos in comments
Documentation: clarify asynchronous actions
Merge pull request #1987 from jroper/1987-update-build-docs

Update documentation for build
Append the value of the vary header instead of overwriting it in the Gzip filter.
Merge pull request #2243 from billthornton/bug/gzip-filter-vary-header

Append the value of the Vary header instead of overwriting it in the Gzip filter.
Merge pull request #2187 from fernandoacorreia/doc-async

Clarify documentation on Action asynchronicity
Merge pull request #2225 from fernandoacorreia/fix-typo-given

Fix typos in comments
Merge pull request #2223 from dhruvbhatia/master

Allow Evolution/Error messages to overflow horizontally