Don't set `content` on `Group`s

[flamingos-cant]

Expansion of the discussion here

When serialising communities to Groups, Lemmy puts the community's sidebar as HTML into the summary field and markdown into the source field. The source, however, is meant to indicate what the content field was generated from. This didn't matter pre-1.0 because Lemmy didn't set content for Groups.

#5120 introduced a new short description for communities, mapped that to summary and changed the sidebar to content. #6152 then flipped this.

This means we now set content and source for Groups, but they don't map to each other like the spec says they should. To mitigate this we could (from my most to least preferable):

1. Don't set content at all, define a new custom property (eg desription) to federate the new short description data.
   This won't break anything and requires very little work to implement.
2. Rename source to a custom property (summarySource)
   This won't break federation as most software doesn't set source and the ones that do need to support converting from HTML-to-MD anyway. This will mean some custom markdown won't work between versions because of limitations with html2md (probably worth looking at htmd at some point).

[dessalines]

If we do the breaking change and just let

name -> display_name / title.
content + source -> markdown / sidebar
summary -> summary / short description.

then, this will be a non-issue.

At most it'll be an issue for a few months as old instances aren't updating.

[Nutomic]

I thought this would cause problems with federating the sidebar, but at least between Lemmy 0.19 and 1.0 it would be okay. This is because we always read the sidebar from source.content if available, so summary and content are usually ignored. It may still cause problems for platforms like Piefed or NodeBB, but at least they will get the short description instead of the full sidebar.