Bump version to 0.7.3

[lpgauth]

Summary

• Bump version to 0.7.3 in shackle.app.src
• Add 0.7.3 CHANGELOG entry (foil 0.1.6 + knot 0.1.2 upgrades, bench/profile tooling removal)

Note: this commit was pushed to #137 but the PR merged before it landed, so it's re-submitted here.