Closes #122: Implement a mechanism to enforce policy before executing branch actions

[jeremystretch]

Fixes: #122

• Introduce configuration parameters for branch action validators
• Add can_$action() methods on the Branch model
• Prevent branch actions from being committed where not permitted by policy
• Dry runs for sync, merge, and revert are still allowed

[arthanson]

There should be tests added for these, but as there isn't a mock plugin in the test suite I'm assuming it will be for later. Maybe add a FR for creating such a test?