Add support for upcoming `source_id` column in the `gf_entry` table

[zackkatz]

Gravity Forms 2.9 adds source_id to the entry table. It stores the ID of the page where the form was embedded.

• $entry['source_id'] is null if on a Form Preview page
• It is the integer the page otherwise

See https://github.com/gravityforms/gravityforms/pull/2880 for implementation details.

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Minimum

• [x] Add an Entry Meta field with "Source ID" name
• [x] Support the link_to_source option from the source_url field
• [x] Add a field option to display either the ID or the page title
• [x] Add a check in the constructor to make sure Gravity Forms 2.9-beta or newer is running (otherwise $entry['source_id'] will not be set).

[rafaehlers]

@zackkatz Is this done? Should we test it?

[zackkatz]

It's done, but the tests fail….

Please do test @rafaehlers.