Allow customisation of bootstrap-datepaicker

[mattpitkin]

There are various configuration options for bootstrap-datepicker for displaying the calendar on summary pages. It might be good to expose some of them to the user in configuration files for gw_summary. An couple of examples would be to expose:

• daysOfWeekHighlighted to allow the user to highlight particular days of the week;
• or, datesDisabled to allow certain dates, for which no page exists, to be disabled.

An application of allowing certain dates to be highlighted, and others disabled, is in #259, which allows a list of given dates to be highlighted, with all the rest disables by default. This makes use of the beforeShowDay option to turn highlighting on for a given selection of dates.