Add option to not show 'Show End Date' option

Works as advertised in my testing.
I'll let Karen review before committing, just to make sure she is aware of this change.

Conceptually this is fine but there are a couple things to tweak:

- I think it's bad UX (and confusing) to have one checkbox that says 'Display all day checkbox' and another one that says 'Disable "Show End Date" checkbox', I think the second should be 'Display end date checkbox' so it matches the first, and the code for what to do with it has to be adjusted accordingly.

- That option should only appear on dates that have an optional end date, in all other cases it should be hidden and set to the right value.

- Changes like this tend to break tests since the field probably has to be set in the tests as well, so we have to make sure the tests still work. That is all the tests listed under 'Date' and the one date test listed under 'Migrate'.

Events with Date-fields can also be imported by Feeds.

I can't tell what #3 means with regard to this issue. The checkbox only applies to fields being managed in the UI, it is a form setting.

at #2, you tell us:

the one date test listed under 'Migrate'

This is ok, but 'Migrate' is not the only Import-module. I also use 'Feeds'.
I have never used the test-modules, so I cannot know, whether 'Feeds' makes problems too.

Thanks for your work and the great modules.

One of the tests in this module is listed under Migrate. There is no test for feeds.

Hi,
In my project, i want end date as optional. I have patched to collect end date as required, optional and the user may decide.

Related Issue and patch Date range needs "open" end date