Load a secret "From File" in "Recover a Secret" flow

[garrettr]

We can't load a secret from a file in the "Recover a Secret" flow.

Motivation: I want a small but non-trivial bug to work on so I can gain some experience with React and the secretshare codebase. Discussed this with @GabeIsman and agreed this was a desired feature that also fits the criteria of a good first bug. He was thinking of refactoring this into a unified "share input" component capable of handling either text input or file input. Since doing that refactor together sounds like a useful educational experience, I am going to start by doing this my (very likely to be naive and unsophisticated) way, and we can revisit refactoring after that.

[GabeIsman]

This has been implemented.