Rename prometheus Vue components

What does this MR do?

1. Renames several Vue components to make their purpose more clear.

   old	new
   ./monitoring.vue	./dashboard.vue and ./graph_group.vue
   ./monitoring_state.vue	./empty_state.vue
   ./monitoring_row.vue	./graph_row.vue
   ./monitoring_column.vue	./graph.vue
   ./monitoring_legends.vue	./graph/legend.vue
   ./monitoring_deployment.vue	./graph/deployment.vue
   ./monitoring_flag.vue	./graph/flag.vue

2. Refactors graph within a group into a flexbox-based layout rather than the unnecessarily complicated mix of nested bootstrap .row and .col-md-6/12 elements. This reduces the complexity of much of the Vue logic.

^ this bit should be moved into a separate MR to make this one less complicated to review.

Are there points in the code the reviewer needs to double check?

This should conform to the Vue style guides

Why was this MR needed?

Style and naming preferences. It was not initially clear to me that the actual graph logic was contained in monitoring_column.vue, for instance.

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

• Tests updated for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

n/a