Add ability to update thumbnail and preview on client

client/src/app/+admin/follows/follows.component.html

client/src/app/core/server/server.service.ts

client/src/app/shared/forms/form-validators/video.ts

client/src/app/shared/forms/markdown-textarea.component.html

client/src/app/shared/misc/utils.ts

client/src/app/shared/shared.module.ts

client/src/app/shared/video/video-edit.model.ts

client/src/app/shared/video/video-thumbnail.component.html

client/src/app/shared/video/video.service.ts

...c/app/videos/+video-edit/shared/video-edit.component.html

...c/app/videos/+video-edit/shared/video-edit.component.scss

...src/app/videos/+video-edit/shared/video-edit.component.ts

...nt/src/app/videos/+video-edit/shared/video-edit.module.ts

client/src/app/videos/+video-edit/video-update.component.ts

...nt/src/app/videos/+video-watch/video-watch.component.html

client/src/app/videos/+video-watch/video-watch.component.ts

client/src/sass/application.scss

server/controllers/api/config.ts

shared/models/config/server-config.model.ts

shared/models/videos/video-update.model.ts