allow cross step snackbar

[tttp]

There are a bunch of cases where a notification (snackbar) are displayed on a step based on the action of the previous step

for instance:

• if double opt-in: display on share (or step after register) a "check your email to confirm"
• if action confirm: same, but different criteria
• coming back from a doi confirm link to the share step

and there are a bunch of less than optimal cases, like the share intro message assumes an action was taken previously (thank you..."), it might not always be the case

so let's set up a generic system to allow a step to set a message to display on the next step