Prepare 0.15.0 release

[russell-islam]

Summary of the PR

Please summarize here why the changes in this PR are needed.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[roypat]

Thanks! It looks like we might be missing some changelog entries, could you add some for #285 and #270 (which is the change we're doing the release for in the first place, haha)

[russell-islam]

Thanks! It looks like we might be missing some changelog entries, could you add some for #285 and #270 (which is the change we're doing the release for in the first place, haha)

Please take another look now.

[roypat]

Looks good, thanks!