feat(api): add plan submit/get endpoints

[hbollon]

I had a unique endpoint /api/plans which is used to submit plan in POST and get ones with GET method:

• GET request: <IP>:8080/api/plans?lineage=<LINEAGE>&limit=<Optional limit> results will be sorted by most recent to oldest
• POST request: <IP>:8080/api/plans with as body:

{
    "lineage": "<LINEAGE>",
    "terraform_version": "0.12.11",
    "git_remote": "https://github.com",
    "git_commit": "#deadbeef",
    "ci_url": "https://github.com/ci",
    "source": "foo",
    "plan_json": <PLAN_JSON_OUTPUT>
}

I also done some tweaks to the plan model:

• added a missing omitempty tag on PlanStateResource.SchemaVersion
• replaced PlanOutputChange by generic Change structure (and adapted json unmarshalling method), this led to the deletion of the unnecessary plan_output_changes table with the new model

This PR depends on #173, will be ready after his merge and a rebase

[coveralls]

Coverage decreased (-0.6%) to 11.597% when pulling 9814af5 on hbollon:api_endpoints into aa7d455 on camptocamp:master.

[raphink]

LGTM

[hbollon]

@raphink can we merge this PR?
So that I can open to review #176

[raphink]

LGTM