Replace API integer API choice values with slugs

[jeremystretch]

Fixes: #3569

Replaces all numeric choices with more human-friendly slugs. For example, the dcim.Site status field changes from

    "status": {
        "value": 1,
        "label": "Active"
    },

to

    "status": {
        "value": "active",
        "label": "Active"
    },

For backward-compatibility purposes, the legacy integer values will still be accepted on write through the v2.7 release cycle.