[1.7] Add composable index templates artifacts (#1156)

[webmat]

Backports the following commits to 1.7:

• Add composable index templates artifacts (Add composable index templates artifacts #1156)
• Move _meta section back inside mappings, in legacy templates. (Move _meta section back inside mappings, in legacy templates. #1186)

[webmat]

@ebeahan For this backport I had to undo the ES 6 fallback stuff, since it wasn't backported to 1.7.

I'll wait for your review on this one

[webmat]

@ebeahan Alright I took some liberties on where innocuous metadata was nested in the original PR for composable templates.

The 1.7 release (and all previous ones) incorrectly had the _meta fields inside the mappings. I wonder if I should modify this backport to not change this, for version 1.7 🤔

What do you think?

[ebeahan]

I tried testing out these changes to the legacy template, but _meta doesn't appear supported as a the top-level field in the legacy index template API. Trying to upload the template with _meta present resulted in this error:

{
  "error": {
    "root_cause": [
      {
        "type": "parse_exception",
        "reason": "unknown key [_meta] in the template "
      }
    ],
    "type": "parse_exception",
    "reason": "unknown key [_meta] in the template "
  },
  "status": 400
}

[webmat]

Oh thanks for testing that 👍 I should totally have done that!

But this means I'll have to fix that in master as well.

[webmat]

I've introduced the #1186 fix in this backport. So as expected, the legacy templates are no longer changing in the diff. Aka _meta is back where it belongs.

Ready for a final review when you're ready @ebeahan

[ebeahan]

LGTM 👍