Remove unused locale messages (#7190)

* 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.

app/_locales/gu/messages.json

@@ -9,24 +9,12 @@
   "about": {
     "message": "વિશે"
   },
-  "accept": {
-    "message": "સ્વીકારો"
-  },
   "account": {
     "message": "એકાઉન્ટ"
   },
-  "add": {
-    "message": "ઉમેરો"
-  },
-  "address": {
-    "message": "સરનામું"
-  },
   "advanced": {
     "message": "વિગતવાર"
   },
-  "available": {
-    "message": "ઉપલબ્ધ"
-  },
   "back": {
     "message": "પાછળ"
   },
@@ -54,12 +42,6 @@
   "connect": {
     "message": "કનેક્ટ કરો"
   },
-  "connecting": {
-    "message": "કનેક્ટિંગ..."
-  },
-  "continue": {
-    "message": "આગળ વધો"
-  },
   "copy": {
     "message": "કૉપિ કરો"
   },
@@ -69,12 +51,6 @@
   "create": {
     "message": "બનાવો"
   },
-  "createDen": {
-    "message": "બનાવો"
-  },
-  "customize": {
-    "message": "કસ્ટમાઇઝ કરો"
-  },
   "delete": {
     "message": "કાઢી નાખો"
   },
@@ -93,9 +69,6 @@
   "fast": {
     "message": "ઝડપી"
   },
-  "fastest": {
-    "message": "સૌથી ઝડપી"
-  },
   "forgetDevice": {
     "message": "આ ઉપકરણને ભૂલી જાઓ"
   },
@@ -119,15 +92,9 @@
   "loading": {
     "message": "લોડ કરી રહ્યું છે..."
   },
-  "min": {
-    "message": "ન્યૂનતમ"
-  },
   "next": {
     "message": "આગલું"
   },
-  "notFound": {
-    "message": "મળ્યું નથી"
-  },
   "origin": {
     "message": "મૂળ"
   },
@@ -158,18 +125,12 @@
   "search": {
     "message": "શોધો"
   },
-  "select": {
-    "message": "પસંદ કરો"
-  },
   "send": {
     "message": "મોકલો"
   },
   "settings": {
     "message": "સેટિંગ્સ"
   },
-  "status": {
-    "message": "સ્થિતિ"
-  },
   "submit": {
     "message": "સબમિટ કરો"
   },
@@ -187,8 +148,5 @@
   },
   "viewAccount": {
     "message": "એકાઉન્ટ જુઓ"
-  },
-  "warning": {
-    "message": "ચેતવણી"
   }
 }