Remove block override dependency on Gutenberg

[bobbingwide]

There are currently major performance problems attempting to use the Site Editor to edit the SB theme. See #6.

In order to determine what's causing these problems I want to be able to test the theme in multiple environments, with fewer activated plugins.

Requirement

The block override logic should work even when Gutenberg is not activated.

Proposed solution

I implemented this for Fizzie. Similar code will be needed in the block overrides for SB.

1. The sb_maybe_override_block() function needed check the currently assigned render_callback function
2. Each block override function that references a gutenberg_ prefixed function should test for its existence before calling it.