Fixed typo "optinal" to "optional" for translation dictionary import dialog and 2 other occurrences

(cherry picked from commit 77a2e4b557)

src/Umbraco.Web.UI.Client/lib/markdown/markdown.editor.js

@@ -1,4 +1,4 @@
-// needs Markdown.Converter.js at the moment
+// needs Markdown.Converter.js at the moment
 
 (function () {
 
@@ -1590,7 +1590,7 @@
     };
 
     // takes the line as entered into the add link/as image dialog and makes
-    // sure the URL and the optinal title are "nice".
+    // sure the URL and the optional title are "nice".
     function properlyEncoded(linkdef) {
         return linkdef.replace(/^\s*(.*?)(?:\s+"(.+)")?\s*$/, function (wholematch, link, title) {
             link = link.replace(/\?.*$/, function (querypart) {

src/Umbraco.Web.UI.Client/src/views/dictionary/import.html

@@ -43,7 +43,7 @@
               <localize key="actions_chooseWhereToImport">Chose where to import</localize>
               <localize key="dictionaryListCaption">dictionary items</localize>.
             </strong>
-            (optinal)
+            (optional)
           </p>
 
           <umb-tree section="translation"