Avoid some moment object creation in modifier generation #1662
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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:
![image](https://user-images.githubusercontent.com/195534/58670813-3d62e300-82f5-11e9-91f8-8b6c0b0559f8.png)
After:
![image](https://user-images.githubusercontent.com/195534/58670824-48b60e80-82f5-11e9-9635-dd41b6ef8574.png)