Prevent entering PMIDs with leading zeros

[lgeistlinger]

It is currently possible to enter a PMID with leading zeros. This should be prevented in the Study form where curators enter this information, as this leads to study duplication:

https://bugsigdb.org/Study_580 (PMID entered without leading 0) https://bugsigdb.org/Study_731 (PMID entered with leading 0)

[lwaldron]

Amazing the corner cases we get once a lot of people start curating. Worth eliminating leading zeroes.

[tosfos]

Should be done now. Please let us know if you experience any issues.

[lgeistlinger]

Looks good! Thanks!