Avoid some moment object creation in modifier generation

[lencioni]

Creating these moment objects is expensive, and we do it a lot in
addModifier/deleteModifier, which get called a lot whenever these day
pickers update. For this particular one, we almost always want the same
value every time it is called, so we can memoize the previous value, a
la reselect.

This reduces the amount of time spent generating modifiers from ~46ms to
~32ms.

Before:

After:

[coveralls]

Coverage increased (+0.03%) to 84.631% when pulling 0e2ed9a on memolast into e3b9c8d on master.