Move 4 component imports out of mixins/pipelines.js

Problem Statement

The Vue mixin at:

• app/assets/javascripts/pipelines/mixins/pipelines.js

Has a very wide scope, which makes it confusing to reason about. Is used by two components:

• app/assets/javascripts/commit/pipelines/pipelines_table.vue
• app/assets/javascripts/pipelines/components/pipelines_list/pipelines.vue

One of the areas that can be easily removed are the components imported by it:

• PipelinesTableComponent
• SvgBlankState (note this may have been removed by #301010 (closed))
• EmptyState
• GlLoadingIcon

Proposal

Remove all imports of components to the mixin and move those imports to the "child" components that use them:

• app/assets/javascripts/commit/pipelines/pipelines_table.vue should import:
  • pipelines-table-component
  • svg-blank-state
  • empty-state
  • gl-loading-icon
• app/assets/javascripts/pipelines/components/pipelines_list/pipelines.vue should import:
  • pipelines-table-component
  • svg-blank-state
  • gl-loading-icon

Not in the scope: Move other logic in the mixin, that could be done separately.