Removed explicit ExecutionContext args

Now that ExecutionContext arguments are implicit (completed in part 2 - #1087), we can get rid of explicitly passed ExecutionContext arguments throughout the rest of Play.

I have closed PR #1133, which also removed explicit EC args, but which also removed the global iteratee EC. After some discussion it was decided that removing the global iteratee EC was a big change that needs some more thought behind it. That's why I closed that PR and provided this PR instead.

Previous parts:
- #807 part 1: #996
- #807 part 2: #1083
- #807 part 3: #1133 (closed)