Add lint rule requiring strict mode

.eslintrc

@@ -15,6 +15,7 @@
       "error",
       { allow: ["warn", "error"] },
     ],
+    "strict": ["error", "safe"],
   },
   "parserOptions": {
     "ecmaVersion": 6,

h/static/scripts/date-util.js

+'use strict';
+
 // cached date formatting instance.
 // See https://github.com/hypothesis/h/issues/2820#issuecomment-166285361
 var formatter;

h/static/scripts/directive/spinner.js

+'use strict';
+
 module.exports = ['$animate', function($animate) {
-  'use strict';
   return {
     link: function(scope, elem) {
       // ngAnimate conflicts with the spinners own CSS

h/static/scripts/filter/persona.js

+'use strict';
+
 /**
  * Parses H account names of the form 'acct:<username>@<provider>'
  * into a {username, provider} object or null if the input does not

h/static/scripts/filter/test/persona-test.js

+'use strict';
+
 var persona = require('../persona');
 
 describe('persona', function () {

h/static/scripts/frame-rpc.js

+'use strict';
+
 /** This software is released under the MIT license:
 
   Permission is hereby granted, free of charge, to any person obtaining a copy

h/static/scripts/test/bootstrap.js

+'use strict';
+
 // Expose the sinon assertions.
 sinon.assert.expose(assert, {prefix: null});
 

scripts/create-github-release.js

 #!/usr/bin/env node
 
+'use strict';
+
 /**
  * Creates a GitHub release for the repository.
  *