Allow optional time in date metadata field

[clojj]

Fixes #257

TODOs:

• use Alternative for that ugly code in parseZettelDate ?
• unit tests

[clojj]

 * [x?]  Drop `created` terminology and retain `date`. Rationale here: [#257 (comment)](https://github.com/srid/neuron/issues/257#issuecomment-672891322)

 * [x]  Change the Zettel field to `zettelDate :: Either Day LocalTime` (which incidentally is isomorphic to `(Day, Maybe TimeOfDay)`. We should not automatically add a time (midnight or midday) if the user has not specified one. Less magic, and more explicit types are a good thing.
   
   * Perhaps make a type alias first: `type DateMayTime = Either Day LocalTime`

 * [x]  Revert the visual regression to how `?timeline` gets rendered; i.e., it should display only day, not the time. However, make it display the full time of the day (if specified) on mouse hover (by setting the `title` HTML attribute).

 * [ ]  Add tests
   
   * Make sure that zettel sorting takes time into account, if specified.

 * [x]  Remove the "Created on:" at the bottom, because that can be confusing if date is not always creation date.

...posted here, just to pick up from the last PR

[clojj]

I guess you were referring to that sorting test in ZettelSpec ?

[srid]

Generally looks good, but will require some changes.

[clojj]

@srid all comments resolved

[clojj]

The JSON API has to be changed or is that wip ?

[srid]

The JSON API has to be changed or is that wip ?

That is automatically taken care of by the Generic Aeson instance, when you modified the Zettel type by changing the field to zettelDate :: Maybe DateMayTime. I added the label only as a reminder for myself, in the context of Cerveau (which stores the JSON in postgres, and will require migration/reset when upgrading production)

EDIT: It is also to remind us that the associated change may impact editor support (emacs, vim) which uses neuron query's JSON output.