deps: Upgrade and move jquery-form from static/third to npm

- Remove `jquery-form` from `static/third` to `npm`.
- Upgrade `jquery-form` to 3.50.0.
- Bump up the `PROVISION_VERSION` to 3.28.

Fixes zulip#1709.

docs/THIRDPARTY

@@ -137,10 +137,6 @@ Files: static/third/jquery-filedrop/jquery.filedrop.js
 Copyright: Resopollution
 License: Expat
 
-Files: static/third/jquery-form/jquery.form.js
-Copyright: M. Alsup
-License: Expat or GPL-2.0
-
 Files: static/third/jquery-idle/jquery.idle.js
 Copyright: 2011-2013 Henrique Boaventura
 License: Expat

package.json

@@ -14,6 +14,7 @@
     "i18next-browser-languagedetector": "0.3.0",
     "i18next-localstorage-cache": "0.3.0",
     "jquery": "1.12.1",
+    "jquery-form": "3.50.0",
     "jquery-mousewheel": "3.1.6",
     "jquery-validation": "1.15.1",
     "perfect-scrollbar": "0.6.15",