Short url: Embed flag is not respected

[flash1293]

Kibana version: 7.9 BC2

Describe the bug: iframe short urls created prior to 7.7 lose the embed flag and are thus rendering with visible chrome

Steps to reproduce:

1. Open Kibana < 7.7
2. Go to dashboard and create iframe short url (will look like this: /goto/sdhfkjsdfhjklds?embed=true)
3. Upgrade to Kibana 7.7 or later
4. Visit short url
5. Dashboard is rendered with visible chrome

Expected behavior:
Dashboard should render in embedded mode.

Any additional context:

This is happening because this change: #58846
In 7.7 and after the embed flag is part of the saved short URL. While this behavior makes sense, it didn't take into account existing short urls with the embed flag outside of the url saved object.

This can be fixed by checking whether there is a embed flag in the server side short url redirect handling and including it into the redirected url.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.