Organize code vertically (by feature)

[Martinsos]

For example, File Upload functionality should be vertically organized, so that file upload / download stuff is all at a single place (ideally in a single directory, both client and server code) and can be easily deleted at one go (I was deleting it manually and that was quite a PITA, hunting it all down through the codebase).
Some stuff will be harder to organize like this and that is fine, but I believe most of the stuff we will be able to organize. For example Admin stuff, that should also be easy to group under one directly.
Same goes recursively, we can group stuff by sub-feature in a bigger feature.

While at it, consider also grouping declarations in main.wasp by feature. For example, File stuff, be it entities, routes, queries or actions, should be grouped together. This makes it easier to delete them or update them all at once, or just get a sense of what is happening with File, what is it about.

This is a big change, so I suggest doing it at the moment (if we have a choice) when we don't have a ton of PR's active, because those will all need to be updated accordingly.

TODO:

• Grouped all file-upload functionality. #170
• Vertical auth #210
• Organize payments vertically #225
• reorganize admin dash #227
• Vertical user #226
• Openai demo app vertical reorg #234
• Leftover vertical org changes #233