CMS-BOT Issue commands

[smuzaffar]

For CMSSW Issues, following commands in first line of a comment are recognized

• +1|approve[d]|sign[ed]|+category: L1/L2's to approve it
• -1|reject[ed]|-category: L1/L2's to reject it
• assign <category>[,<category>[,...]]: Any L1/L2's to request signatures from other categories
• unassign <category>[,<category>[,...]]: Any L1/L2's to remove signatures from other categories
• hold: L1/all L2's/release manager to mark it as on hold
• unhold: L1/user who put this PR on hold