Cast firstDay input to number for moment >=2.11

Starting with moment.js 2.11.0, the first day of the week (`dow`) needs
to be supplied as a number type (e.g. `0` for Sunday). Supplying a
string (e.g. `"0"` for Sunday) will result in incorrect week number
calculation. This Calendar app sets FullCalendar's `firstDay` option,
which in turn sets moment.js' `dow`.

Both moment.js and FullCalendar have indicated that they will not be
accepting or sanitizing strings (see moment/moment issue 2900 and
fullcalendar/fullcalendar issue 3014).

Note that FullCalendar is supplied by this Calendar app, but that
moment.js is supplied by core. Both ownCloud core and Nextcloud core
currently still depend on moment 2.10.

Also note that week number functionality for this Calendar app is
still under development (see owncloud/calendar issue 262 and
fullcalendar/fullcalendar issue 3024).

However, when using the week number functionality in combination with
a manual upgrade of core's moment to version 2.13.0, week numbers were
indeed calculated incorrectly (see screenshots in the pull request for
this commit).

This commit contains a preventive fix that adds unary plusses to the
input for FullCalendar's `firstDay` option, so that any string is cast
to number. It can be applied now already and will prevent problems when
core upgrades its moment.js dependency to a higher version.