[MM-36747] Spellchecker custom urls (cherrypick to TS) (#1669)

* [MM-36747] Allow users to specify spellchecker url for downloading dictionaries

* fix settings keys

Co-authored-by: = <=>

src/common/config/index.ts

@@ -229,6 +229,11 @@ export default class Config extends EventEmitter {
     get useSpellChecker() {
         return this.combinedData?.useSpellChecker ?? defaultPreferences.useSpellChecker;
     }
+
+    get spellCheckerURL(): (string|undefined) {
+        return this.combinedData?.spellCheckerURL;
+    }
+
     get spellCheckerLocale() {
         return this.combinedData?.spellCheckerLocale ?? defaultPreferences.spellCheckerLocale;
     }