[Page attachments][Outlink] Links not visible in Instagram in-app browser

### Description

This was [originally reported](https://wordpress.org/support/topic/swipe-up-links-dont-show-on-instagram-browser/) by a user of the Web Stories WordPress plugin.

They found that page attachment links / outlinks (the story is using ``) were not visible when viewing a story in Instagrams in-app browser. I was able to confirm this myself.

Interestingly, in my testing the links suddenly appeared when closing the story and reopening it.

Here's a demo video showing the issue:

https://user-images.githubusercontent.com/841956/130607138-e078b6fb-1117-4f46-a9bc-029d1e880e98.mov

See how the "Shop" link isn't visible at first but appears once the story is closed and opened again.

The story appears just fine when viewing in Safari on iOS itself, so it seems limited to Instagram's in-app browser.

### Reproduction Steps

1. Create a web story with page attachment links / outlinks
2. Link to the story on Instragram, for example in the bio or in an Instagram story
3. Click to view the story so it opens in Instagram's in-app browser
4. Notice the absent page attachment link

### Relevant Logs

_No response_

### Browser(s) Affected

_No response_

### OS(s) Affected

iOS

### Device(s) Affected

iPhone

### AMP Version Affected

2108052321001

Start by reproducing the missing page attachment link using the listed reproduction steps on an iPhone in Instagram’s in-app browser, then compare with Safari and the supplied demo behavior. Trace the AMP page attachment/outlink behavior involved; done means the link is visible on the initial story load without needing to close and reopen it.

Written by the indexing model from the issue text.