Instagram Gallery Block: Stop automatically connecting blocks without access token

[Copons]

Changes proposed in this Pull Request:

• Only attempt to automatically connect to Instagram if the Instagram Gallery block has an access token.

Previously

On mount, IG blocks automatically attempted to connect to Instagram.
If they had a token stored, they validated it and, if valid, used it.
If they didn't have a token, they fetched it and, if existing, used it

This had the intended side effect of automatically connecting new blocks inserted by users with valid tokens.
But it also had the unintended side effect of... automatically connecting blocks inserted by other users and left "empty" (not connected, without access token) in a post.

Now...

On mount, IG blocks only automatically attempt to connect to Instagram if they have a token stored.
They will validate it and, if valid, use the token.

"Empty" blocks don't automatically connect anymore.
Instead, they will show the normal "Connect to Instagram" button.
But before sending the user over to Instagram to authorize the connection, we first check if they have a valid token.
If they do, the block will then, finally, automatically use the token, and the user won't be sent to Instagram anymore.
If they don't, they'll proceed to Instagram to create a new token as usual.

Testing instructions:

• Insert an Instagram Gallery block, and connect it to Instagram.
• If you have a valid Instagram token, make sure the block embeds the gallery straight away.
• Otherwise, make sure you are redirected to Instagram to authorize the connection.
• Insert another block, and make sure it shows the "Connect" button regardless.
• Connect the second block too (again: no Instagram authorization should be needed).
• Now disconnect one of these two blocks.
• Make sure both blocks are disconnected.
• Connect one of them: make sure you're sent to Instagram to authorize the connection.
• Make sure the other block is not connected automatically.

Proposed changelog entry for your changes:

• N/A

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: June 2, 2020.
Scheduled code freeze: May 26, 2020

Generated by 🚫 dangerJS against 1a3860d

[Copons]

@scruffian apologize for having moved this back to WIP without notice!

In 8058f64 I've actually removed the auto-connect flow, which was kinda the whole point of the PR.
It wasn't used anymore and the execution shouldn't have really reached it.

I've only kept the "auto-disconnection", which is used when the user disconnects a block, to automatically disconnect all other blocks using the same token.

[scruffian]

LGTM and works as advertised

[jeherve]

This works well in my tests.

[Copons]

r207600-wpcom