Display the remediation cost of each issue

When computing the technical debt measure, each remediation cost computed on an issue should be saved on this issue to be displayed in the Web interface.

This requires:

• to add a new column on the "Issues" table
• to update the Web interface to display this information properly
  • Note: in this case (= when there's a remediation on an issue), the Web interface should also display the requirement linked to this remediation cost

We should also display the technical debt on file headers on issues code viewer.