Split out editing venues from editing schedule

[kr-matthews]

The styling is a bit off with the "Venues + Add a venue" and the text at the top of each tab being a bit close to the content below (formerly there was an accordion).

Now each folder has an index and a (single) component subfolder with another index, so it might be worth bringing those subfolder contents up and having a single index (maybe as a follow-up PR?).

There's some minor duplication now, since everything was duplicated then trimmed away, mostly in the 2 top-level index files.

I removed a scss file because it didn't appear to be doing anything, but I'm not 100% confident in this.