Add post scenario enablement hook

Problem/Motivation

It would be useful if modules could respond to the enablement of a scenario, as right now the Scenarios API only allows for hooks to be run after individual migrations have finished.

Proposed resolution

Add a new hook, hook_scenarios_post_enable(), which will enable this functionality.

Remaining tasks

Review the patch.

User interface changes

None.

API changes

hook_scenarios_post_enable() has been added and documented.

Data model changes

None.