Prefix navbar engine selection with a :

Fix #4221

Auditors: @bridiver

Test Plan:  ':a' searches with amazon but  'a' doesn't.

js/data/searchProviders.js

@@ -9,54 +9,72 @@ module.exports = { "providers" :
       "image" : "https://www.amazon.com/favicon.ico",
       "search" : "https://www.amazon.com/exec/obidos/external-search/?field-keywords={searchTerms}&mode=blended",
       "autocomplete" : "https://completion.amazon.com/search/complete?method=completion&q={searchTerms}&search-alias=aps&client=amazon-search-ui&mkt=1",
-      "shortcut" : "a"
+      "shortcut" : ":a"
     },
     {
       "name" : "Bing",
       "image" : "https://www.bing.com/favicon.ico",
       "search" : "https://www.bing.com/search?q={searchTerms}",
       "autocomplete" : "https://api.bing.com/osjson.aspx?query={searchTerms}&language={language}&form=OSDJAS",
-      "shortcut" : "b"
+      "shortcut" : ":b"
     },
     {
       "name" : "DuckDuckGo",
       "image" : "https://duckduckgo.com/favicon.ico",
       "search" : "https://duckduckgo.com/?q={searchTerms}&t=brave",
       "autocomplete" : "https://ac.duckduckgo.com/ac/?q={searchTerms}&type=list",
-      "shortcut" : "d"
+      "shortcut" : ":d"
+    },
+    {
+      "name" : "GitHub",
+      "image" : "https://assets-cdn.github.com/favicon.ico",
+      "search" : "https://github.com/search?q={searchTerms}",
+      "shortcut" : ":gh"
     },
     {
       "name" : "Google",
       "image" : "https://www.google.com/favicon.ico",
       "search" : "https://www.google.com/search?q={searchTerms}",
       "autocomplete" : "https://suggestqueries.google.com/complete/search?client=chrome&q={searchTerms}",
-      "shortcut" : "g"
+      "shortcut" : ":g"
+    },
+    {
+      "name" : "Stack Overflow",
+      "image" : "https://cdn.sstatic.net/Sites/stackoverflow/img/favicon.ico",
+      "search" : "https://stackoverflow.com/search?q={searchTerms}",
+      "shortcut" : ":s"
+    },
+    {
+      "name" : "Mozilla Developer Network (MDN)",
+      "image" : "https://developer.cdn.mozilla.net/static/img/favicon32.png",
+      "search" : "https://developer.mozilla.org/search?q={searchTerms}",
+      "shortcut" : ":m"
     },
     {
       "name" : "Twitter",
       "image" : "https://twitter.com/favicon.ico",
       "search" : "https://twitter.com/search?q={searchTerms}&source=desktop-search",
-      "shortcut" : "t"
+      "shortcut" : ":t"
     },
     {
       "name" : "Wikipedia",
       "image" : "https://en.wikipedia.org/favicon.ico",
       "search" : "https://en.wikipedia.org/wiki/Special:Search?search={searchTerms}",
-      "shortcut" : "w"
+      "shortcut" : ":w"
     },
     {
       "name" : "Yahoo",
       "image" : "https://search.yahoo.com/favicon.ico",
       "search" : "https://search.yahoo.com/search?p={searchTerms}&fr=opensearch",
       "autocomplete": "https://search.yahoo.com/sugg/os?command={searchTerms}&output=fxjson&fr=opensearch",
-      "shortcut" : "y"
+      "shortcut" : ":y"
     },
     {
       "name" : "Youtube",
       "image" : "https://www.youtube.com/favicon.ico",
       "search" : "https://www.youtube.com/results?search_type=search_videos&search_query={searchTerms}&search_sort=relevance&search_category=0&page=",
       "autocomplete": "https://suggestqueries.google.com/complete/search?output=chrome&client=chrome&hl=it&q={searchTerms}&ds=yt",
-      "shortcut" : "yt"
+      "shortcut" : ":yt"
     }
   ]
 }