Add the ability to archive a task

[joshsmith]

Problem

We need to add the ability to archive a task.

• Add an archived column to tasks

[begedin]

Needs

• add migration which adds :archived field, type of :boolean, null: false, default: false
• add field to Task model
• cast :archived as part of the Task.update_changeset/2
• modify tests for Task.update_changeset/2

Total estimate ~2 hours