Create migration to set all existing task and comment timestamps

[begedin]

Problem

We have new time stamps added due to github integration, paired with helper fields indicating the origin of the change

field :created_at, :utc_datetime
field :created_from, :string, default: "code_corps"
field :modified_at, :utc_datetime
field :modified_from, :string, default: "code_corps"

• All fields where created_at is nil should be set to the value of inserted_at
• All fields where created_from is nil should be set to code_corps
• All fields where modified_at is nil should be set to the value of updated_at (or is it edited_at?)
• All fields where modified_from is nil should be set to code_corps

The same migration should be applied to comments as well, since they have the same field