Submit Bigram: Detect and merge bigrams when uploading an image for an existing SB

[bobbingwide]

The Submit Bigram page used the [bw_new] shortcode to create a new post with the title of the bigram.
It doesn't check for an existing post.
When an image is being uploaded and the bigram was in the original SB list, or has been sampled or seen before then it makes sense to check if the bigram already exists. If it doesn't have a featured image then, rather than create a new one, we can reuse that post.

Requirements

• Ability to update an existing post using [bw_new] or similar.
• Merge post content from the original and new posts.
• Set the featured image on the original post.
• Don't actually create a new post.

[bobbingwide]

For an example see https://seriouslybonkers.com/bigram/simply-beauty/
https://seriouslybonkers.com/bigram/simply-beauty-2/

The first post was created by searching for Simply Beauty while logged in.
The second was created when I uploaded the image.

Workaround

• Search for and edit the original post in preference to using Submit Bigram.

Note: Submit Bigram doesn't often work from my mobile phone. Not sure about editing existing posts on the mobile either. It certainly doesn't work using the WordPress Mobile App, which doesn't support CPTs.

[bobbingwide]

This enhancement requires changes to the [bw_new] shortcode in oik-fields.