Small cleanup of the front matter functions

[jeanphilippegg]

In this pull request:

Add denote--retrieve-front-matter-*-from-content functions

These are just like denote-retrieve-front-matter-* functions, but they work on a content string instead of a file. (Maybe there would a way to remove the code duplication further.)

With them, we can remove the two functions denote--get-{title/keywords}-line-from-front-matter.

Clean up denote--format-front-matter

• The date parameter is now a date object and denote-format-front-matter handles its formatting through denote-date, like other components.
• The identifier line is now handled like the other components in md front matters.
• Removed denote--format-front-matter-{title/keywords} as they do not need a function of their own. All front matter components are handled within denote--format-front-matter.

Some internal support of signatures in front matter

Two internal functions handle signature lines in the front matter. There are still a few things to adjust and signatures are still not included in the default front matter templates. So they do nothing by default for now.

[jeanphilippegg]

I just pushed a small change to make sure the signature will be sluggified in the front matter.

It is ready to be merged!

[protesilaos]

Merged, thank you!