Add Validation for the Store / Cover Date

[bpepple]

Describe the bug
Currently the site always any valid date to be used as a Cover or Store date. This can lead to some issues having dates like date(218, 5, 30), which while technically a valid date, obviously an error for a comic. In the example given that date when written to xml would not be a valid date.

Expected behavior
Adding some date validation to the Issue form to not accept years less than 1900 would solve the problem