main.html, en.json: Add built by coala

add the built by coala in main.html footer, add built-by
property in en.json

Closes #22

Author: pshreya2181

static/i18n/en.json

@@ -6,6 +6,7 @@
     "gci-copyright": "Google Code-in and the Google Code-in logo are trademarks of Google Inc.",
     "gitlab-copyright-1": "The GitLab logo is licensed under",
     "gitlab-copyright-2": "by GitLab Inc.",
+    "built-by": "Built by",
     "orgs-no-listed": "Organization(s) with no listed leaders",
     "passed": "$1% passed, approximately $2 before task claiming ends",
     "seconds-ago": "$1 {{PLURAL:$1|second|seconds}} ago",

templates/main.html

@@ -209,7 +209,8 @@ <h3>
     </div>
     <footer>
       <small data-i18n="gci-copyright">Google Code-in and the Google Code-in logo are trademarks of Google Inc.</small><br />
-      <small><span data-i18n="gitlab-copyright-1">The GitLab logo is licensed under</span> <a href="https://gitlab.com/gitlab-com/gitlab-artwork/blob/master/LICENSE">CC BY-NC-SA 4.0</a> <span data-i18n="gitlab-copyright-2">by GitLab Inc.</span></small>
+      <small><span data-i18n="gitlab-copyright-1">The GitLab logo is licensed under</span> <a href="https://gitlab.com/gitlab-com/gitlab-artwork/blob/master/LICENSE">CC BY-NC-SA 4.0</a> <span data-i18n="gitlab-copyright-2">by GitLab Inc.</span></small><br />
+      <small><span data-i18n="built-by">Built by <a href="https://coala.io/">coala</a>.</span></small>
     </footer>
     <script src="{{assets.main-js}}"></script>
   </body>