Schema: rationalize dates, fixes #162

[rhiaro]

Overview

• What does this pull request do?

Simplifies dates as detailed in #162

• Interest startDate and endDate, PEP Status startDate and endDate, Entity foundingDate and dissolutionDate, and formedByStatute date are more strict - only date (YYYY-MM-DD) is valid; timestamps and partial dates are no longer valid.
• Person birthDate and deathDate are more strict - year (YYYY), year and month (YYYY-MM) and year, month and day (YYYY-MM-DD) are valid, but timestamps are no longer valid.
• Annotation creationDate, PublicationDetails publicationDate are less strict - valid as date or date-time (partial dates are still not valid).

Updates tests/date and adds a couple of extra tests.

• How can a reviewer test or examine your changes?

Check the new schema descriptions and definitions against the spreadsheet

• Who is best placed to review it?

@kd-ods

(Closes/Relates to) issue: #162

Translations

• New or edited strings appearing as a result of this PR will be picked up for translation
• I've notified the translation coordinator of any new strings that will need
  translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

• I've thought about how and when this needs to be released. See:
  https://openownership.github.io/bods-dev-handbook/standard_releases.html
• I've updated the changelog, if necessary.
• I've updated reference.rst to reflect any changes to schema structure or naming.
• I've added or edited any other related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md

[kd-ods]

@rhiaro - Is \d definitely valid regex here? I can't get your pattern to match dates using https://regex101.com/ . Poss an issue with me or the validator!

Otherwise this PR looks ready to go to me.