Make Scratchpad the default user script editor.

See: https://developer.mozilla.org/en-US/docs/Tools/Scratchpad

Fixes #1688
Show parse-time script errors when installing.
Localize string in install dialog.

Refs #1695
Localize strings in parseScript module.

Refs #1695
Localize strings in script module.

Refs #1695
Localize various orphaned strings.

Fixes #1695