Resolve#30350
The action unit of mirrors and templates should be disabled by default.
This PR adds `DEFAULT_MIRROR_REPO_UNITS` and
`DEFAULT_TEMPLATE_REPO_UNITS` options to allow users to specify default
units for mirrors and templates.
Thanks to @lng2020 for the
[idea](https://github.com/go-gitea/gitea/issues/30350#issuecomment-2053942243)