Make date formats more configurable

[philgyford]

Stemming from philgyford/django-hines#205

Although we added the SPECTATOR_DATE_FORMAT setting, this isn't used when we're generating a span of dates for readings. e.g. "1 February 2017 to 2018", "1-6 February 2017" or "February to March 2018".

So I think we need a bunch of settings to be used by spectator.reading.templatetags.spectator_reading.reading_dates(). Initial guesses, with defaults:

SPECTATOR_DATE_FORMAT = "%-d %B %Y"  # "3 September 2017", already exists

SPECTATOR_DATE_FORMAT_MONTH_YEAR = "%B %Y"  # "September 2017"

SPECTATOR_DATE_FORMAT_DAY_MONTH = "%-d %B"  # "3 September"

SPECTATOR_DATE_FORMAT_YEAR = "%Y"  # "2017"

SPECTATOR_DATE_FORMAT_MONTH = "%B"  # "September"

SPECTATOR_DATE_FORMAT_DAY = "%-d"  # "3"

SPECTATOR_PERIOD_FORMAT_SHORT = "{}–{}"  # Like "3–5 September 2017"

SPECTATOR_PERIOD_FORMAT_LONG = "{} to {}"  # Like "3 September 2017 to 3 March 2018"

But... while I want to be able to make all day-specific periods for Hines like "2019-10-28", doing the above alone wouldn't let us generate links for each date to that day's page.