Add support for CMB2 custom fields

[DevinWalker]

Issue Overview

When I was doing work on Zapier and Recurring I noticed our nice welcome texts are missing in the setting tabs:

Branch 1.7: Nice welcomes :)

Branch 1.8: No welcomes :(

[DevinWalker]

@ravinderk I know you're not able to reproduce...from debugging I was able to narrow it down to get_filtered_addon_settings: https://github.com/WordImpress/Give/blob/release/1.8/includes/admin/settings/class-settings-cmb2-backward-compatibility.php#L259

When the $settings for in the fields look OK, but then nothing is output. I'd like to dig into why this is happening for Zapier. I can't find any difference between the other Add-ons in how it registers it's settings.

[ravinderk]

@DevinWalker CMB2 backward compatibility breaks because WordPress updated there hook system in version 4.7.

[07-Dec-2016 14:44:35 UTC] PHP Notice:  Undefined index: function in wp-content/plugins/Give/includes/admin/settings/class-settings-cmb2-backward-compatibility.php on line 509

For ref: https://make.wordpress.org/core/2016/09/08/wp_hook-next-generation-actions-and-filters/

[DevinWalker]

@ravinderk it's still not working for me :(

[DevinWalker]

I just reviewed again and my Add-on settings pages appear working nicely albeit minor CSS that needs to be addressed with Zapier:

Closing.