Date and datetime inputs and defaults #2254

Work in progress #2254 

- [x] use default only for new entries - not for existing entries
- [x] not setting a default, should not default to today (you can click calendar anyway)
- [x] Fill in time input field, when fieldtype has changed from [date to datetime](https://gist.github.com/GDmac/5dccbec77e125c5e126b)
- [x] clicking calendar icon should [not] set date/datetime #2260 
- [x] Styling option <del>class: widedate</del> `variant:inline` for datepicker form rows
- [x] Add note to examples and docs
  https://github.com/bolt/bolt-docs/pull/118