Update status messages

Problem/Motivation

Currently, the DefaultStyleguide plugin displays the three type of status messages individually. But status-messages.html.twig doesn't work that way. All types of status messages are rendered together as a group separated into types. Therefore, the way the style guide renders messages isn't relevant to possible real-world scenarios. For instance, you won't be able to get an accurate representation of how the message types are spaced.

Furthermore, the plugin currently displays only one message per type. The default template will display multiple messages of the same type in a <ul>. So this is another deficiency.

Proposed resolution

• Add a group of status messages that have different types.
• Display both single and multiple message per type.
• Consider removing the individual status messages. I don't see any value in keeping them versus a group. Alternatively, repurpose them for displaying the single or multiple messages (whatever the group doesn't display).

Remaining tasks

• Add the new items.
• Decide what to do about the single messages.
• Add screenshots of the proposed changes to the UI changes section below.

User interface changes

API changes

Data model changes