general: WTForms 2 compatibility fixes

* Support for *WTForms* in *Flask-Admin* is coming thanks to (mrjoes/flask-admin#676). I have tested deposit Article form and basic functionality seems to work.
* If you have custom forms in the overlay plea read [what's new](https://wtforms.readthedocs.org/en/wtforms2/whats_new.html) first (cc @lnielsen-cern, @jalavik, @jmartinm, @egabancho).

#### How to install

```console
$ pip install -e git+https://github.com/arsgeografica/flask-admin@f836bf46ba9a2c8c709cd7d47cdc92e13a56efa5#egg=Flask-Admin
$ pip install --upgrade wtforms
```