Fix installation of metapackages from repositories

[HebaruSan]

Problems

• @ProgrammerFailure reported problems installing modpacks from https://github.com/ProgrammerFailure/MyCKANMetaData, contained in this metadata repo:
  https://github.com/ProgrammerFailure/MyCKANMetaData/archive/main.tar.gz
  
  The changeset appears, but when you confirm it, the install is skipped.
• Once that was fixed, I got HTTP status 415 errors trying to install a mod that was indexed based on its source archive
  
• Then once the modpack was finally installed, I noticed that the changeset wasn't updating when I toggled the Auto-Installed checkbox

Causes

• Metapackages and dependencies were being filtered out, so the install flow was quitting on an empty changeset
• GitHub's API documentation now says to set the Accept header to application/vnd.github+json to download a source ZIP rather than a value that corresponds to a ZIP file type (!!!)
  https://docs.github.com/en/rest/repos/contents#download-a-repository-archive-zip
  
• The Auto-Installed checkbox has gone through a number of changes relating to the ability to remove individual items from the changeset screen, which at some point probably severed the link between it and refreshing the changeset, but I was not able to pinpoint precisely when this occurred.

Changes

• Now it's fine to put metapackages in a changeset; they'll just be skipped during the parts of installation that involve downloading and unzipping files. An InstalledModule will still be added to the registry even though no files are installed for a metapackage, so it can be upgraded when there are updates in the upstream repo. Immediate dependencies of a metapackage will be marked as non-auto-installed by default so the metapackage does not have to remain installed to keep the mods (since this is how "install from .ckan" currently works).
• Now the GithubTransformer will set download_content_type to application/vnd.github+json when x_netkan_github.use_source_archive is true, so clients will pass the mime type that GitHub foolishly requires for source ZIPs
• Now GUIMod.SetAutoInstallChecked raises the PropertyChanged event with an argument of IsAutoInstalled, which ManageMods receives and handles by refreshing the changeset