* Switch to using string literals for locale keys
Various message keys were being specified with a string template
instead of a string literal. They have been switched to use string
literals so that the script for detecting unused messages can find
them.
* Remove unused locale messages
A number of unused locale messages have been removed - probably
leftover from old UI elements that have since been removed.
The `verify_locale_strings` script has been augmented to search the UI
for string literals, and match those against the locale message keys in
the `en` locale. Any messages without a corresponding string literal
are assumed to be unused.
The script has also been updated with an optional `--fix` parameter,
which will automatically delete any unused messages from locales.
148 unused messages were found in this case, out of a total of about
650 messages. Another 70 messages are _potentially_ unused and require
further investigation, but weren't as easy to rule out because they
were found in string literals.
* Remove additional unused locale messages
The following messages were more difficult to rule out because they
were present as string literals in the UI. They do appear to be
unused as locale keys though.
* eslint: Check for unused function arguments
* eslint: Ignore unused '_' in argument list
Also allow any number of '_' e.g., '__' or '___' which is to be used sparingly
* Remove and rename unused arguments
