Fixes #853 time slice reset on layer switch

When switching layers the time slice selectors were being defaulted back
to the max and min values of the extent.
Ensure the isDirty check can be performed correctly based on changed field
Refactor time range label into custom class
Merge branch 'master' into issue-853
Changes as per review
Merge branch 'master' into issue-853
Added custom UTC format method to moment prototype

DRY up calls to our displayed UTC date format
Merge pull request #915 from aodn/issue-853

Issue 853