Add category message

[siliconfeces]

This PR makes the toast after adding an archive to a category more helpful.
Before:

After:

More interestingly, it also adds support for backend code (aka perl) to return the success message that appears in toasts.

[Difegue]

Looks good at a glance -- Could you edit the example API response in the Docs as well? Thanks!

[siliconfeces]

Looks good at a glance -- Could you edit the example API response in the Docs as well? Thanks!

Done, but not sure how to check what the rendered documentation looks like so I might have fucked something up.

[Difegue]

Well, there's no way for me either to know how the doc will look like 'til it lands in dev or master since that's on GitBook's hands, but I'm fairly sure it'll look fine like this.

Mergin' this 👍

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