Handle .srt subtitles

...s/+video-edit/shared/video-caption-add-modal.component.ts

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

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

package.json

server.ts

server/controllers/api/videos/captions.ts

server/helpers/captions-utils.ts

server/helpers/custom-validators/video-captions.ts

server/initializers/constants.ts

server/initializers/installer.ts

server/lib/cache/abstract-video-static-file-cache.ts

server/lib/cache/videos-caption-cache.ts

server/lib/cache/videos-preview-cache.ts

server/models/video/video-caption.ts

server/tests/api/check-params/video-captions.ts

server/tests/api/videos/video-captions.ts

server/tests/fixtures/subtitle-bad.txt

server/tests/fixtures/subtitle-good.srt

server/tests/utils/videos/videos.ts

yarn.lock