Server: add job scheduler to transcode video files

config/default.yaml

config/production.yaml.example

config/test-6.yaml

server.js

server/initializers/checker.js

server/initializers/constants.js

server/lib/jobs/handlers/index.js

server/lib/jobs/handlers/video-transcoder.js

server/lib/jobs/job-scheduler.js

server/models/job.js

server/models/video.js