Gutenberg/free media library

[etoledom]

Fixes wordpress-mobile/gutenberg-mobile#720

This PR implements the iOS Bridge code needed for Stock Photos (Pexels) and wordpress-mobile/gutenberg-mobile#1265

gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#1539

Note:

• The "Free Photo Library" is the same as used on Aztec.

• I decided to make the Free Photos Library multi-selectable, even though we haven't activated this feature on iOS yet. I can keep it single selectable if we decide that this is better.

• There are some few small changes on the bridge media picker/upload methods, so it would be good to test all options on all media related blocks.

• I noticed that we could improve some things in the JS side, but opted to keep this PR focused on adopting the current implementation to avoid the need of Android changes.

To test:

Image Block:

• Add an Image block.
• Check that the "Free Photo Library" option is there.
• Open the Free Photo Library.
• Choose one (or many) images.
• Check that all images selected are added.
• Check that you can select only images in all other options.

Video Block:

• Add a video block.
• Check that "Free Photo Library" is NOT in the options.
• Check that you can only select videos in all other options.

Media&Text:

• Add an Media&Text block.
• Check that the "Free Photo Library" option is there.
• Open the Free Photo Library.
• Choose one (or many) images. (Multi-select is also possible on Android)
• Check that all images selected are added. (Extra images will be added as Image Block)
• Check that you can select images and videos in all other options.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[SergioEstevao]

Looking good, just need to update the release notes!

[etoledom]

Thank you!