DL-1778 Play 2.6 upgrade

Maintenance

Upgraded CTR frontend to latest supported version of Play.

Checklist

[x] I've made every effort to commit high quality, clean code and I have executed relevant static analyses to be sure
[x] I've included appropriate tests with any code I've added (Unit, Integration, Acceptance etc.)
[x] I've executed the acceptance test pack locally to ensure there are no functional regressions
[x] I've added my code using logical, atomic commits, squashing as appropriate - including the JIRA issue number in the commit message
[x] I've run a dependency check to ensure all dependencies are up to date