Bump ember-data from 2.18.2 to 3.15.0

Bumps [ember-data](https://github.com/emberjs/data) from 2.18.2 to 3.15.0.
- [Release notes](https://github.com/emberjs/data/releases)
- [Changelog](https://github.com/emberjs/data/blob/v3.15.0/CHANGELOG.md)
- [Commits](emberjs/data@v2.18.2...v3.15.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]