Add a helper to generate file URLs

[walkah]

Right now, people have to generate permalink URLs "By hand" ( a la ipfs.runfission.com/ipfs/${cid}/${filename} ) which isn't impossible, but could be easier. We should also ideally have two versions:

1. Generate a link to a specific CID (this will be "frozen" in time )
2. A version that tracks the current version of a given file.

[bmann]

For reference, we're likely moving to files.runfission.com and could also do per-user links like boris.files.runfission.com/p/Screenshots/some-screenshot.png

This may need more certificates? Putting this note here as reference :)

[matheus23]

We've got a demo app for the different kinds of file paths: https://benevolent-senior-pink-yeti.fission.app/

Also, we've had a talk post that's related to this issue: https://talk.fission.codes/t/linking-to-public-files/1716

And, I think we should include something like this function: https://github.com/fission-suite/dashboard/blob/8ef6e7031b8f6f20f2927e7e6c16349d9ba24ec9/src/Javascript/index.js#L334-L339

[icidasset]

Implemented on next branch.