Set defaultDate for Pikaday to avoid "date jump"

[cpjolicoeur]

Since we do not use/include moment.js in Torch (to
avoid adding more package dependencies and bloat),
Pikaday uses a function similar to the following to set
the default date if no defaultDate param is provided.

defaultDate = new Date(Date.parse(field.value))

In Torch, we set the input field value in the ISO-8601 format
of YYYY-MM-DD. The ISO-8601 spec should treat strings without
timezone specifiers as local time.

However, depending on the browser and ES version (ES5 vs ES6, etc...)
Date.parse will assume UTC. So, in the above, Date.parse is returning
UTC values, which then are passed to the new Date constructor which
returns a date in the local timezone, and depending on what the local
timezone is, can result in the actual date display being off by 1 (in
either direction).

With this change, we now manually create a default date ourselves to
pass as the defaultDate argument to Pikaday to avoid any issue like
this in the future.

[Close #241]