Add link to TranslateWiki.net in the footer

To make it easier for people to know they can help translate
the UI.

Author: samwilson

i18n/en.json

@@ -32,6 +32,7 @@
     "report-issue": "Report an issue",
     "language": "Language:",
     "language-switch": "Switch language",
+    "help-translate": "Help translate this tool",
     "langs-placeholder": "Leave blank for automatic language detection.",
     "langs-param-error": "The following {{PLURAL:$1|language is|languages are}} not supported by the OCR engine: $2",
     "loading-message": "Performing transcription...",

i18n/qqq.json

@@ -39,6 +39,7 @@
 	"report-issue": "Link text in the footer for the issue-reporting link.",
 	"language": "Label for the language-switching dropdown.",
 	"language-switch": "Button text for the language-switching dropdown.",
+	"help-translate": "Link text for a link to the tool's translation page on TranslateWiki.net.",
 	"langs-placeholder": "Placeholder text for the language input field.",
 	"langs-param-error": "Error message displayed when invalid language(s) are submitted.\n\nParameters:\n* $1 – number of invalid languages\n* $2 - the list of invalid languages\n\nOCR is a common abbreviation in English for \"Optical Characters Recognition\".",
 	"loading-message": "Loading message displayed when the transcription is being performed",

templates/base.html.twig

@@ -87,6 +87,7 @@
                             {% endfor %}
                         </select>
                         <input type="submit" value="{{msg('language-switch')}}" class="btn btn-default">
+                        <a href="https://translatewiki.net/wiki/Special:Translate?group=wikimedia-ocr&language={{ lang() }}" target="_blank">{{ msg('help-translate') }}</a>
                     </div>
                 </form>
             </footer>