Adding a SIP note without date throws error instead of providing a helpful message

[toraritte]

Defer working on this until issue #2 "Make input date methods uniform" is resolved. By providing a pre-populated date input and by only offering a selection of date in certain ranges, this issue can be closed as well, but make sure to test it first. (Undefined input may still be possible if one edits the page's HTML directly, but that is not considered normal usage.)

[toraritte]

Depends on:

• Make date input methods uniform #2