There was a bunch of warnings repeated over and over during spec runs:
```
/var/www/discourse/lib/tasks/release_note.rake:3: warning: already initialized constant DATE_REGEX
/var/www/discourse/lib/tasks/release_note.rake:3: warning: previous definition of DATE_REGEX was here
/var/www/discourse/lib/tasks/release_note.rake:5: warning: already initialized constant CHANGE_TYPES
/var/www/discourse/lib/tasks/release_note.rake:5: warning: previous definition of CHANGE_TYPES was here
```
* DEV: Improve rake `release_note:generate` date handling
A commit-ish value like HEAD@{2021-01-01} is based on the **local state** of HEAD on that date. It does not use dates attached to commits.
Instead, the rake task now detects date-like strings and supplies them to `git log` via the `--after` and `--before` flags
* Skip printing plugin when there are no changes found
A list of skipped plugins is printed on a single line at the end of the output
`UX` is the officially supported prefix per https://meta.discourse.org/t/19392, but sometimes `UI` is used instead. We should still include those commits.
Limit git log output to the first line of the commit message
when generating the list of commits in a release. Some commit messages
are including the names of all commits that were squashed, resulting
in duplicate and confusing lines in the release notes.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
