follow up to ControlledGate implementation

[ajavadia]

follow up to #2862:

• write release notes (added release notes for controlled gate class (#2862). #3465)
• delete unused imports in test/python/circuit/test_controlled_gate.py (PR Removing unused import modules from test_controlled_gate.py #3487)
• add circuit.to_gate() shorthand (replace q_if with control #3455)
• replace q_if with control (replace q_if with control #3455)
• [ ] bug: cannot get gate.to_matrix() for a gate obtained from instruction_to_gate(circuit_to_instruction(circ))
• bug: base_gate is a class, not an object, so it throws away the params (done in PR Params in controlled gates #3454)
• the optimize_1q_gates should be able to handle optimizations where the chain of single-qubit gates are controlled by the same control lines

[1ucian0]

Closing this issue since the last bullet is tracked by #3563