Add 'Expires' to genform, using bulma-calendar

[joker314]

Closes #41

Here is a picture of the new entry into the generation form. It doesn't look too different to the other inputs, which I think is quite nice:

In theory, this PR could be merged now and the functionality is all there. But first I want to go through and fix stuff up:

• consider using Intl JS API for formatting dates
• decide how to handle timezones
• remove the 'Validate' button in the calendar which causes the form to prematurely submit
• investigate accessibility (especially keyboard accessibility) with the calendar. This Bulma extension is incredibly useful but e.g. tabbing into it seems to not be obvious
• code cleanup: make 'Expires' less of a special case
• in the defaults.html, add a comment just for the Bulma extension bit
• consider using the JS file with all the bulma extensions, since a future PR will be using the form validation one too
• there's a typo in the description for 'Expires' (periodically is misspelled)

[joker314]

Let's redo this with <input type="date"> together with a <input type="time">. Using "datetime" or even "datetime-locale" could be bad for browser support (Firefox 👀)