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: D400 First line should end with '.', not 's'
4: I102 copyright year is outdated, expected 2014 but got 2013
28:1: F401 'TextAreaField' imported but unused
31:1: F999 redefinition of unused 'current_user' from line 26
42: D101 Docstring missing
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137: D203 Expected 1 blank line *before* class docstring, found 0
137: D204 Expected 1 blank line *after* class docstring, found 0
137: D400 First line should end with '.', not 'm'
137: D200 One-line docstring should not occupy 3 lines
160: D102 Docstring missing
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