Add comment filtering by reply count

client/src/app/shared/video/sort-field.type.ts

.../app/videos/+video-watch/comment/video-comment.service.ts

...videos/+video-watch/comment/video-comments.component.html

...videos/+video-watch/comment/video-comments.component.scss

...p/videos/+video-watch/comment/video-comments.component.ts

server/initializers/constants.ts

server/models/utils.ts

server/models/video/video-comment.ts