Fix login fail when opening plugin (self-managed)

What does this MR do?

Ensures useSelfManagedGitlab is set when the plugin UI sandbox gets data from Figma.

Also adds various comments to login.vue.

Related issue: #36 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Code review guidelines (@mention a member of the Create:Knowledge Frontend engineering team for review)
• [-] Added tests for new functionality
• [-] Code review guidelines