Add ability to query for lists of draft entries

[justlevine]

Gravity Forms doesn't have a public API for accessing a list existing draft entries, instead requiring you to know the draft token.

A database query needs to be written to grab all of them, so we can support both gravityFormsDraftEntries and including draft entries in the existing gravityFormsEntries.

[justlevine]

We need to think hard about the auth restrictions on this, i.e. who can query for lists of draft entries.

Natively, you need the resume_token, which is only displayed to the user filling out the form (although they theoretically can share the link), but not even Super Admins can see a list of draft entries.

I see a use-case for showing users a list of all their own draft entries, but the ramifications of making this data available to admin roles needs to discussed. Mainly: is there a reasonable expectation that users who save a submission for later will have their unfinished data kept private from the site administrators?

That said, the Partial Entries Add-on does add unsubmitted entries to the backend entry list, I believe without explicitly indicating such on the form. So maybe we don't need to care either?

@tinytoolbox @natac13 - I know Harness relies heavily on draft entries, and I'm sure you have some valuable real-world insights.