Bump version
Update grammar versions
Update changelog
Add signing script
Update the release-checklist with public release procedure
Merge branch 'release/0.28.0'
Merge tag '0.28.0' into develop

* Enhancements
  * Several content enhancements
  * [#110] Upgrade to API 20
  * [#360] Switch to grammar v3.1 allowing optional progress header
  * [#365] Pre-load sequences to avoid waiting for BK tree load in autoList questions
* Bug fixes
  * [#301, #302, #303] Design and icon fixes; also better space usage when sequences have no intro