Exclude built assets from git

[duncanmcclean]

This pull request removes the built Control Panel assets from the repository and adds a release workflow which builds them at release time, matching how we handle assets in statamic/cms.

Keeping compiled assets out of version control has a few benefits:

• It's more secure — compiled assets are effectively unreviewable, so this prevents anyone slipping malicious code into a pull request via the dist files.
• It avoids merge conflicts whenever multiple branches touch the front-end.
• It brings this repository inline with our other repositories.

Releasing

To tag a release, run the "Create Release" workflow from the Actions tab and give it a version number. It'll build the assets, commit them onto the tag, and create the GitHub release using notes from the CHANGELOG.md this PR also adds.

[duncanmcclean]

@jasonvarga I'm assuming this is the plan for addons going forward? (copying how we do it in core with the manually triggered workflow)

[jasonvarga]

Looks good but the two secrets only exist on cms.
They could probably be changed from repo secrets to org secrets.
You'd also need to update the bot to allow access to mailables-viewer. Right now its just cms.