Refactor toast pattern into behaviour

Based on this MR review suggestion:

thought: I wasn't aware of this pattern (along with flash[:toast] = ...). It's... weird 😅

Thinking out loud (not for this MR) I wonder if this should instead be a behavior, and extracted to an async chunk that's only fetched if there are any .js-toast-message elements at page load 🤔

That would remove the need for different entry points to do this... Though, it's only 5 at the moment 🤷

Proposal

• Add a new initToastMessages function to app/assets/javascripts/behaviors/toast.js, and run it from behaviors/index.js.
• Function should querySelectorAll('.js-toast-message'), and bail early if no messages were found. Otherwise it should
• Asynchronously import showToast from '~/vue_shared/plugins/global_toast'
• Call showToast for each message

We can then clean up the various places in the codebase which manually do these steps, e.g.:

• ci_cd/show/index.js
• pages/shared/wikis/wikis.js
• pages/admin/geo/index.js
• settings/ci_cd/show/index.js
• projects/settings/analytics/index.js

(probably not a conclusive list!)