Add in applied feedback messages

[karmatosed]

This is an exploration of the idea suggested by @hedgefield and builds on from the messages work in #3457.

Let's consider having feedback messages when something is applied. Some good cases for this would be:

• Links
• Captions
• Buttons..

Possible placing for these:

In top bar: this is the least elegant solution in my opinion

Beside the action: not sure this scales for all cases

Underneath the action: this possibly is a better format

Let's research and consider before we design

• Where should the message go?
• Are there any existing patterns in core we can use for this?
• If there are no existing, what are some examples design wise of this in other applications?

[mtias]

Why do we need messages for these actions? I think messages should be used for important async calls, like updating and saving a post or a global block. The success message for applying a link should be that the link shows.

[hedgefield]

For links this isn't such a problem, but the Button block does not distinguish between an entered URL and an applied URL. Pressing Enter or the Apply button does not give any feedback. That's confusing. A whole message might be a bit intense, but at least the button should change styling/become hidden/display a checkmark to show that what you did had an impact. See #3427 (comment) for context.

[karmatosed]

Let's close and look at auditing messages as a group.