Add admin tool for renaming/deleting backlogs
Actual time check only for bugs in a sprint

Added admin parameter to enable actual work time check only when bug is
in a sprint.

Fixes #13
Improve estimated time change restirictions

Only apply original estimate change restriction to committed sprints
and provide a more meaningfull error message when user is not allowed
to change the original estimate.
Do not calculate work if sprint capacity is not defined