fix: make export key a subtle link, add view to VIEWS array

- Moved 'Export private key' from prominent button row to a small
  muted text link at the bottom of the address detail view
- Added 'export-privkey' to the VIEWS array in helpers.js — this was
  the cause of the blank view (showView toggled all known views but
  didn't know about export-privkey, so it was never unhidden)

src/popup/views/helpers.js

@@ -31,6 +31,7 @@ const VIEWS = [
     "approve-site",
     "approve-tx",
     "approve-sign",
+    "export-privkey",
 ];
 
 function $(id) {