Support for Joplin-Style EML Metadata Headers

[elsiehupp]

Is your feature request related to a problem? Please describe.

I currently use Evernote. I would like to be able to escape from Evernote. The notes I have in Evernote have metadata I would like to preserve, such as manually configured (i.e. backdated) note creation dates for content created outside of Evernote.

Describe the solution you'd like

Nextcloud Notes could support the header format used by Joplin, which is based on the EML format. Even if Nextcloud Notes wouldn't immediately support all of the metadata fields, it could still preserve any existing ones on imported notes. The headers would be hidden by default, as in most GUI email clients, but they could be viewed and edited in plaintext via a supplementary interface. Metadata fields could be incrementally added to the GUI, but the existence of the EML-header backend would allow for the incorporation and preservation of metadata yet to be implemented in the user interface.

Describe alternatives you've considered

The main alternative is just using Joplin or sticking with Evernote.

Additional context

This issue is related to #164, but the difference is that I'm proposing the implementation and support of a specific metadata format already used by a major FOSS Markdown note-manager application. Implementing this issue might also fix #584, though the exact method of doing so would be open to debate. It could also fix #299, though actually including tags in the GUI would be separate from supporting tags under the hood. Again, the main thing here is supporting and preserving arbitrarily extensible imported metadata, regardless of whether or not individual fields are implemented in the user interface.

Ultimately it might be nice to have full database cross-compatibility (or at least JEX compatibility) with Joplin, but I recognize that implementing that is a much taller order. The main reason I keep circling back to Joplin is that it specifically supports importing Evernote ENEX exports, and in so doing preserves metadata initially created within Evernote. If Nextcloud Notes were to support Joplin's metadata format, this would create an indirect migration path from Evernote. Nextcloud Notes could also potentially borrow Joplin's ENEX-import module, but I recognize that that's also a much taller order. (The import module is written in TypeScript, though, so porting it would be more straightforward than if it were written in some other language.)