Feature request: Simplified upload/download for simple mods

[Goober5000]

Not every mod necessarily needs its own description, splash page, and configuration. If a user just wants to make a single mission that runs under an existing mod, all of this is unnecessary and redundant.

From Discord discussion:

Goober5000: It would be nice to have a separate Knossos section with a list of single missions and mods they run under. And, conversely, each full mod could have a list of independent single missions that could be run under it. This would also keep the mod listing tidier, as 50 single missions wouldn't require 50 individual mod tiles.
This would require a bit of Knossos UI work, as well as some fsnebula database enhancements, but it could really minimize the work required for FREDders to publish single missions

ShivanSpS: it would need a new mod type, lets call it, "mission" because im very inspired today, and thats about it, and dependencies it would just be a single one pointing to the mod id with the minimal version it runs on.
What else it needs? a search function? thats easy since we have a new mod type and a target mod id (when can use the parent field to store that since its already on mod minimal)

Goober5000: It would need a new Knossos section specifically for single missions. They would appear in this section instead of in the main mod listing. (They would also be listed on the mod they run under.) It would also need a new, simplified upload dialog, that would require only a name, a description, the mission file, and the mod it uses.
The single missions would be downloaded into the "missions" folder of the mod they run under

ShivanSpS: mmmmm this sounds very close to adding steam workshop to mods.
If we are going to do that is not better to just allow any kind of simple downloable content for the mod instead of just missions?

Goober5000: Hmm, theoretically, this makes a lot of sense. But practically, "any kind of downloadable content", if it is multiple files, would be tricky to manage, especially if it modded the mod, so to speak. At least missions are guaranteed not to affect the parent mod

ShivanSpS: this is not really that hard, it just a more limited mod that has 1 package, no dependency, it still needs versioning control i think, that gets installed to the target mod mission folder and hardlinked/copied to all present versions. Actually i think its better to have them somewhere else and just have them be added to the -mod cmdline i belive due to posibility of having multiple versions.
The only changes in nebula would be validation, no db changes are needed
Add a new mod type that is accepted in pre-fight and uploads, and make sure the field "parent" can point to any ID since in this case it will be used to point to the target mod
The only thing im worried about is that, in the end its still a mod and needs an id, but i guess that using the name plus some randomly generated characters would be enough. Well the details can be talked in the issue