Make Frozen contexts non-experimental

Frozen contexts are a useful way of making past courses available but maintaining their state. They is useful, but unused by may as it remains experimental.

This feature was introduced with some trepidation as it relies on capability modification (write capabilities --> Off). Knowing that this would work for both core and contributed plugins is something difficult to determine through automated testing or even QA testing, given the code coverage affected. So the feature was added as experimental and has been that way for several releases to this point.

This feature should now become non-experimental for the following reasons.

• The feature has been bravely turned on at many institutions (including my own school) and while this has turned up a few capability oddities in assignments and importing (which have been fixed), it has generally worked. Interestingly, these problems were problems without freezing, but freezing brought them to light.
• Leaving the feature as experimental inhibits further detection of issues. Now that people have been using this feature in the real world, these issues should not be dramatic. It will also force contributed plugin developers to maintain their code in light of this feature.
• Even if it were non-experimental (on by default) it still requires people to apply this state to categories, so it's harmless unless people choose to use it.