Allow editor links to work inside an iframe or with CSP that prohibits other protocols

[lastobelus]

restrictions for apps running in a third-party iframe

[lastobelus]

for example, Shopify app

[RobinDaugherty]

Thanks @lastobelus! Might it be possible to implement something in Javascript that can change the link attributes when CSP will disallow it from opening? Unfortunately opening a new tab on every click of the editor link seems like a poor compromise. At the least, let's make this configurable so that it can be enabled in applications that need it.

[RobinDaugherty]

I've changed this to use an environment variable, BETTER_ERRORS_INSIDE_FRAME. If that's set, the target attribute will be present. And I added specs to cover this new behavior.