deposit: WTForms version to be <2.0

* Fixing WTForms to <2.0 as Flask-WTF doesn't support it yet.

* Updating Flask-WTF to 0.9.5.

* Updating WTForms-Alchemy to 0.12.4.

* Fixing the build where _unset_value was not found.

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>