[META][UX] Start using the new 'info' message type.

[klonos]

In #3155 where this feature was implemented, we have identified some success (status) messages that would be better as info message instead. This issue if a follow-up to convert these, and perhaps other ones in core.

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PR by @klonos: backdrop/backdrop#2318

[klonos]

...ok, initial PR up for review. I have only worked on messages on the Project Installer.

[klonos]

...I just filed another PR, in which I explore the option of adding some of the Project Installer messages in the Form level. The idea basically is to have messages with problems/info related to the form be closer to the thing they refer to, and to separate them from any possible "global" messages. Here's a before:

...and an after:

PS: shamelessly plugging #3329

[klonos]

...I am making this a meta-issue.