Add Multisite Global Media integration

[jaymcp]

Ref: #13
Ref: #658

Requires Jay's fork of MGM - https://github.com/jaymcp/multisite-global-media

Steps to test:

1. on the "primary" site (i.e. the site declared as the MGM site) - EN, in our case
2. create a post
3. add the following blocks, configuring each with an image:

• humanity theme hero (override the featured image)
• core image
• core cover
• core gallery
• core text-media
• humanity section
• humanity slider
• humanity action
• humanity custom card
• humanity background media

4. add a video to the humanity theme hero block
5. save the post
6. using the MultilingualPress metabox, copy the post to a target site
7. choose the option that copies the content to the remote post
8. save the post
9. view the target post on the remote site
10. each of the images & video(s) within the added blocks should have had their image IDs prefixed with the MGM prefix (100000, in our case)