Restrict issues from being added to sprint if unestimated

Problem Definition

Currently, issues that are not estimated can be added to sprints regardless. When these are added and users change the estimation during sprints, scope gets changed and it also affects charts that are generated for each sprint.

Suggested Solution

Restrict issues being added to sprint if they are not estimated.

Why this is important

It would ensure that scope changes are minimal and agile reports would reflect the sprint better.

Workaround

None at the moment.