Revisit the datetime definitions in the posts

[kokkonisd]

Currently, the post files define the date and time on which a post was published in the following way:

1. By parsing the date out of the name of the file (e.g. YYYY-MM-DD-my-post.md);
2. By parsing the date key from the post file's frontmatter (if present).

We could have the date key override the date provided in the name of the file, but I can't think of a legit usecase for that (there is no good reason for wanting to have a different date on the filename than the actual date displayed on the site). So what the date key is doing, currently, is providing additional information on the datetime (specifically, time and timezone). That means that you have to re-specify the exact date as the one in the filename, plus the time and the timezone.

This seems redundant, and also annoying (you might want all your posts to be UTC); a better solution would be to get rid of date and have two optional keys, time and timezone, which would provide further information on the post.

As an extra thing, we could potentially allow to define a "default" timezone in the config file; to be examined.