#447920: Ensure there is a context selector for the "terms" argument and context converters so it can be used with Views.

#416298: Provide a mechanism so modules such as Hierarchical Select can cooperate with delegator form overriding.

Allow restricting contexts via access control to allow the add content UI to remove irrelevant content.

Expand the context to allow the plumbing for replacing placeholder contexts with real contexts for preview.

Implement context keywords which also use the converters. Add converters to user, fix taxonomy converters.