This change finally sort of puts the parts together: We take a maildir,
render all its note messages as standalone HTML, extract the attachments
alongside and finally generate a global index page linking all notes.
The new executable and mnote-html are both contained in the same image
and we dispatch the right functionality based on argv[0].
Change-Id: I5a5bdbfaca79199f92e73ea4a2f070fa900d2bc4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5113
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
