Gitea and Gitlab allow for filtering push events based on the branch
name, so we assume that webhook payloads don't need to be filtered based
on the branch in these cases. Github doesn't allow for this sort of
filtering, so we have to specify a default branch to filter on.
This commit switches Hugo to a webhook-based building process, with
support for Github, Gitlab, and Gitea hooks (including local versions of
Gitea) initially. In addition, Hugo-based sites are now intended to be
served under a single volume, with ingress configuration pointing to
sub-paths into the volume.
Documentation for webhook setup and NGINX proxy configuration is still
underway, and will be filled in later.
