Improve customizability of planning bug lists
Better handling of bugs with multiple blocked

Place the bug added to buglist under the first bug on the list that it
blocks.
Sprint and Backlog create/update/delete fixes

* Fix Backlog->remove_from_db()
* Wrap the changes in transactions so that in case of failures, stray
  pools or things like that are not left behind
Escape HTML in buglist field values
Add 'additem' event to buglist widget

This is for allowing external customizations of the buglist items after
creation.
Don't show backlog if team has no backlogs
Remove reduntant code from SprintController
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
Check user only when estimated_time is actually changed