NOTE: This PR is stacked on #756 and should be shipped after that is
merged.
This PR enables the existing aggregate relationships work (see #725,
#731, #756) by default by removing the experimental flag it used to be
disabled behind.
The new OpenDD schema changes that were added are also unhidden so that
they are visible in the OpenDD JSON Schema.
V3_GIT_ORIGIN_REV_ID: cfd86d8a9ea61887ccf0f1a5d08bdcc3dda59cdc
## What
This PR introduces a changelog file, `changelog.md`.
Any PR that is not simply a technical refactor should include a relevant
entry in this file.
Additionally we also simplify the pull request template. The template
used to contain a section for a changelog entry, which is now rendered
irrelevant.
V3_GIT_ORIGIN_REV_ID: 00881d86ffe87c4c0584b88b960837543dde34b7
