Add toast messages to the dismissal reason workflow.

Problem to solve

In #10364 (closed) the designs required toast messages to relay to the user what was going on. However, this was deemed to be outside the scope of the MVC.

We need to be sure to add these in to help provide feedback and context to our users.

Please see the original issue for more information, but I've put the relevant information for this below:

Intended users

• Sasha, Software Developer, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#sasha-software-developer
• Sam, Security Analyst, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#sam-security-analyst

Proposal

Toasts should be fired when a user:

• Dismisses a vulnerability
• Adds a comment to a dismissal*
• Edits a dismissal comment*
• Deletes a dismissal comment*

*this functionality does not currently exist: #10364 (closed)

We can utilize the toast component in GitLab UI but it will require a little tinkering since we need to fire the toasts from the Vuex store rather than a component method.

Designs

Dismissed (truncated)

Added

Edited

Deleted