Add "Closing invalid tasks" section

[sobolevn]

Some invalid tasks may appear:

• duplicates
• something that is not needed anymore
• or some other cases

So, this task should be:

• closed
• labels should be removed
• estimate should be removed
• assignee should be removed
• heading should be updated with [Invalid]
• comment why this task is invalid should be created

Only architect can close such issues. Matrix should also be updated.