Note that this also adds a new `.ContentWithoutSummary` method, and to do that we had to unify the different summary types:
Both `auto` and `manual` now returns HTML. Before this commit, `auto` would return plain text. This could be considered to be a slightly breaking change, but for the better: Now you can treat the `.Summary` the same without thinking about where it comes from, and if you want plain text, pipe it into `{{ .Summary | plainify }}`.
Fixes#8680Fixes#12761Fixes#12778Fixes#716
