Add bootstrap vue's modal to gitlab-ui

Scenarios of modal's in GitLab

• Dynamic content modal
  • One modal DOM element with dynamic data loaded across multiple button triggers
• 1 to 1 modal
  • One modal DOM element mapped to one button trigger
• Button in HAML, Modal in Vue

Current implementations of modal's in GitLab

• gl_modal.vue - 11 instances in CE
  • Dynamic content modal (4 count)
  • 1:1 (3 count)
  • button in HAML (4 count)
• deprecated_modal.vue - 7 instances in CE
  • Dynamic content modal (1 count)
  • 1:1 (1 count)
  • button in HAML (5 count)