Improve/Fix JSON Schemas

[m-mohr]

Related Issue(s): #826, #816, #624

Proposed Changes:

1. Align label extension JSON Schema with rest of the extension schemas Inconsistent extension schema format #624
2. Put stac_extension entries into extension schemas instead of core spec schemas stac_extensions in STAC Catalog / Collections #826 JSON Schema: Move stac_extensions enum to extension schemas? #816
3. Make checksum extension schema also validate collections and catalogs.
4. Clarify in checksum extension that in can be used in collection assets, but can't be used in Item assets
5. Merge "Changed" sections in CHANGELOG

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.