Prefetch xpost suggestions in Gutenberg

[guarani]

Addresses wordpress-mobile/gutenberg-mobile#2602

This PR prefetches the suggestion data that populates the Xpost suggestion UI. Without this, the user experiences a network delay when first inserting an Xpost suggestion in the block editor.

Expected behavior

RPReplay_Final1609258379.MP4

Note: The throttle introduced in #15558 is a 60 second throttle. This means that the user will still experience the network delay if they wait for more than 60 seconds between opening the editor and inserting an Xpost suggestion.

To test

What you'll need: Two or more WordPress.com sites that are capable of xposting (https://wordpress.com/p2/ can be used to create xpost-capable sites). It's best if you can test with a site that has lots of Xpost suggestions (10 to 20), to ensure that the network delay would be noticeable if present.

Xpost in Gutenberg

1. Open the block editor
2. In a Paragraph or any other rich text block, type + and verify that the list of Xpost suggestions is shown immediately (no network delay should be noticeable)
3. Switch to a different site from within the editor
4. Type + (or long press the @ toolbar button and select Xpost) and expect the list of suggestions to be shown immediately

Note: There is an issue which @antonis first reported in #15139 (comment) where sites that don't have Xposts suggestions (e.g. non-P2 sites) show the suggestions UI for an instant while the network loads. I didn't address that here because I had difficulty updating the Gutenberg capability when switching between sites while in the the editor. I created wordpress-mobile/gutenberg-mobile#2942 to address this in a later PR.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading it from AppCenter here with build number: 39833. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[guarani]

Adding you here @fluiddot since you mentioned you'd like to review/test this. If you're not comfortable reviewing the code, a test is still much appreciated 🙇 and you can just leave a comment instead of an approval.

[jleandroperez]

✅ Suggestions delay is gone
✅ Switching to another site also prefetches suggestions

Looks good! sir!!

[fluiddot]

It works perfect, nice work 🎊 !

Minor issue: The only thing I noted which I don't know if it's by design is that when you have only the symbol +/@ in the text input and press backspace it hides the x-post UI but doesn't remove the character from the paragraph block. So if you want to trigger the UI again you have to remove the symbol and input it again.

[guarani]

Thank you!

Minor issue: The only thing I noted which I don't know if it's by design is that when you have only the symbol +/@ in the text input and press backspace it hides the x-post UI but doesn't remove the character from the paragraph block. So if you want to trigger the UI again you have to remove the symbol and input it again.

Thanks for reporting that @fluiddot! I think this is a bug that's already in production (tested on iOS and on the Android beta) so I created wordpress-mobile/gutenberg-mobile#2943.