Support multiple FarmDateTimeForm components on a single page

[paul121]

• Allow a custom dateLabel to replace the "Date" label of the component. This allows for multiple FarmDateTimeForm components to be added and labeled for their respective uses eg: "Start Date" and "End Date"
• Allow the date, hour and minute input fields to be empty if the provided timestamp is null. This improves behavior for forms where the date is optional - currently a null timestamp is displayed as Dec 31 1970 in the form. Once the user provides input, a new timestamp is provided.
• Add an id to all form elements so that that multiple FarmDateTimeForm components have unique form elements on the same page. This fixes an issue where the AM/PM radio input was not unique to a single component. (@jgaehring I believe I implemented this correctly? Let me know if not!)

[paul121]

@jgaehring I just added another commit that allows the required property to be false if the provided timestamp is null. This lets me add a "Clear" button to my Date Range form that makes timestamp = null. Without this change the date field would highlight red after clicking "Clear".

Because this all depends on sending the timestamp as null, I don't believe this is breaking in any way. Although it is a rather naive way of making the field optional - is there a better way to make the entire FarmDateTimeForm component optional?

[jgaehring]

Looks great!

Allow the date, hour and minute input fields to be empty if the provided timestamp is null. This improves behavior for forms where the date is optional - currently a null timestamp is displayed as Dec 31 1970 in the form. Once the user provides input, a new timestamp is provided.

So, I'm a little on the fence about this. Originally, the idea was to restrict null or other invalid timestamps because this would be rejected by the server (see #158), but I can see how you wouldn't necessarily want to impose that restriction all the time.

Because this all depends on sending the timestamp as null, I don't believe this is breaking in any way. Although it is a rather naive way of making the field optional - is there a better way to make the entire FarmDateTimeForm component optional?

I think it might be better to have another prop, maybe something like :required="false" but which would default to true. That seems safer to me, but would still allow for flexibility if you wanted to disable it.

[jgaehring]

Added another commit with the required prop. Gonna merge this in and push it out with 0.6.1 (#392).

Thanks @paul121! :clap:

[paul121]

Great! Thanks @jgaehring !