Update readme and stable tag for 0.7.3 release

[goldenapples]

• Adds release notes for 0.7.3
• Bumps asset version number, stable tag, and tested up to version
• Rebuilds .pot files

[goldenapples]

Would love a review of these release notes, and a second check to see if there's anything else ready to go into this release.

[goldenapples]

(The build will pass once #772 is merged.)