Show plot UDF fields on the plot detail page

Fixes #971

Stewardship fields are created for both trees and plots during
instance creation but the template was only displaying collection UDF
fields for the tree model, not the plot model.
Handle collection UDF date fields

The UI was using a simple text box for date fields within collection UDF
fields. I added a date picker to the field and modified the backend to
accept a date without a time as a valid value.

NOTE: This is an intermediate improvement to the date handling on
collection UDF fields. It should be further improved by displaying dates
in a local-appropriate format but posting them to the server in the ISO
format the back end expects.
Collection UDF styling on plot detail

I altered the styling to more closely match the styling of the rest of
the fields on the plot detail page.
Correctly filter collection udf values

Previously, all the collection UDF values for a model across all
instances of the model were being shown on the detail page for a plot
due to the fact that the code used all() rather than filter()
Show planting site instead of plot via filter

There are several places throughout the application when we need to
convert a model name into a title displayed on a page. We are trying
to replace all instances of "plot" with "planting site" so while I was
working on editing collection UDFs I created a reusable filter to
handle the conversion.
Tweak the "no tree" styling on the plot detail page

The text was jammed up against another section header and the easiest
way to improve the look was to convert the span into a div with a
bootstrap "well" class.
Merge pull request #980 from jwalgran/fix/stew-add-and-save

Update collection UDF viewing and editing