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
```
1: D100 Docstring missing
23:5: F401 '_unset_value' imported but unused
25:1: F403 'from abstract import *' used; unable to detect undefined names
26:1: F403 'from author import *' used; unable to detect undefined names
27:1: F403 'from date import *' used; unable to detect undefined names
28:1: F403 'from doi import *' used; unable to detect undefined names
29:1: F403 'from file_upload import *' used; unable to detect undefined names
30:1: F403 'from issn import *' used; unable to detect undefined names
31:1: F403 'from journal import *' used; unable to detect undefined names
32:1: F403 'from keywords import *' used; unable to detect undefined names
33:1: F403 'from language import *' used; unable to detect undefined names
34:1: F403 'from notes import *' used; unable to detect undefined names
35:1: F403 'from pages_number import *' used; unable to detect undefined names
36:1: F403 'from publisher import *' used; unable to detect undefined names
37:1: F403 'from record_id import *' used; unable to detect undefined names
38:1: F403 'from title import *' used; unable to detect undefined names
39:1: F403 'from wtformsext import *' used; unable to detect undefined names