Closes #201: Branch cloning

[jeremystretch]

Closes: #201

Implements the ability to pull changes from one branch into another by replaying the change records from the source branch onto the destination.

• Add related_branch ForeignKey on BranchEvent model
• Implements a pull() method on the Branch model to effect the replaying of changes
• Introduces a view & REST API endpoint for "pull" operations (consistent with sync & merge operations)
• Introduces the pre_pull & post_pull signals
• Add pull_validators configuration parameter

[arthanson]

I think there is a bug here - if you do pull it does not pick up synced changes from the other branch. After noticing this I use the following test and ran into a error:

1. create branch A
2. create branch B
3. in main create a new site "site-main"
4. activate branch A
5. sync branch A (so it picks up "nsite-main")
6. create a device on branch A using "nsite-main"
7. switch to Branch B
8. Pull from Branch A
9. Error.

I think there are other errors as when I created the device in A it auto-created some power ports from the device type I created, the error seems to be coming from those:

{
    "log": [
        "Pulling changes from branch aaa into bbb",
        "Found 10 changes to pull",
        "Applying change 16: tdevaaa created by ahanson using schema_branch_uardeem4",
        "Creating device <DeserializedObject: dcim.Device(pk=107)>",
        "Applying change 17: Input created by ahanson using schema_branch_uardeem4",
        "Creating power port <DeserializedObject: dcim.PowerPort(pk=82)>",
        "{'device': ['device instance with id 107 does not exist.']}"
    ]
}

The device I created (using demo data) was:
Name: tdevaaa
Role: Access Switch
Device type: AP7901
Site: nsite-main