Refactor file management pattern

We have a few places where the user can manipulate "files":

- map edits in the main traffic sim
- named routes in the bike network tool
- bike network extensions (really just map edits, but presented in a different UI)
- story maps (niche/experimental tool but still)
- soon, the modal filter edits in the LTN tool (maybe that'll be map edits underneath, maybe not)

We have no consistent handling today. Some shared concerns / features for these things:

- creating a new / blank file
- saving / autosaving
- naming them (and when do we force a name? and how to assign automatic names?)
- renaming files
- making copies of files
- deleting files
- prompting to save / delete before you navigate away and lose the state
- **sharing** - a new in-progress feature needed ~immediately for #743
- browsing files
- and in the route tool, we even have a quick way of scrolling through all the routes

Step 1 is figure out what the ideal UX of file management even should be. Possible inspirations:
- figma
- excalidraw
- streetmix

Then it'd be awesome to refactor code to handle this stuff consistently!