[A] government agency may only deny a public opportunity for free speech if it can demonstrate a `clear and present danger' of substantial injuries to person or property. That burden is even higher where the government agency is a public university seeking to impose a prior restraint on student speech and assembly on its campus. The university setting is traditionally a `marketplace of ideas' and students should be allowed to speak, rally, debate, and protest as part of their own maturation as well as part of the normal political process.The right of the people to peaceably assemble and to speak freely any viewpoint, however unpopular, cannot be allowed to yield to the speculative apprehension of police and municipal authorities that some unpleasantness or even damage to property and injury to persons might possibly occur. It is the duty of the municipality and the police to prevent such occurrences and to protect the expression of viewpoint