Fix publication year form field functionality.

#5816 

This PR addresses https://github.com/openscholar/openscholar/issues/5816#issuecomment-40226148.

1. The validation of that field takes place in the submit. Only numbers are allowed:
![selection_075](https://cloud.githubusercontent.com/assets/4497748/2689121/2f82dcfa-c2e7-11e3-8884-ea91e3c7579a.png)

2. The JS will now disable the buttons when a year is supplied. If a radio button is selected the numrical year part of the field will be empty.