Remove file drag drop 😟

[fonsp]

Reverts #707

It has not been working well for a long time, and there is no clear path to make it work well.

Small problems:

• Dragging cells between notebooks was broken
• Cell drag and drop broken triggers PNG file upload #1684
• The CSS looked like a quick hack

Larger problems, also see #950

1. Where do we store the file? Right now we create a directory for the notebook assets, but this makes reproducibility tricky. Renaming and relocating was difficult. If we were a cloud service like nextjournal.com, then we could store files online and solve all of these problems, but for a local notebook it is more difficult.
2. The generated code imports a package, but if you import the same file type twice, then the package is simply imported twice, triggering a MultipleDefinitionsError. See File drag-and-drop functionality - put imports into separate Pluto cell #1275
3. Dropping a file into a cell did not make much sense, except when dragging an image into markdown, but that did not work.

Possible solutions are:

1. Embed a DataSets.toml in the notebook file (just like Project and Manifest) and use a base64 driver to store files directly in the notebook file. (Or become a cloud service.)
2. Insert the package import in another cell if it does not exist yet.
3. Smarter UI, maybe a centralized "files" panel with all uploaded files, and some inline CM widget to reference them.

We can always revisit this, and revert this PR to continue working on the feature.

Fix #1275 fix #1684 fix #950

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="remove-file-drag-drop")
julia> using Pluto

[j-fu]

Yeah, now I can move cells in chrome. Tanks!