Add user notification base code

package.json

server.ts

server/controllers/api/users/index.ts

server/controllers/api/users/my-notifications.ts

server/controllers/api/videos/abuse.ts

server/controllers/api/videos/blacklist.ts

server/controllers/api/videos/comment.ts

server/controllers/api/videos/index.ts

server/controllers/feeds.ts

server/controllers/tracker.ts

server/helpers/custom-validators/misc.ts

server/helpers/custom-validators/user-notifications.ts

server/initializers/constants.ts

server/initializers/database.ts

server/lib/activitypub/process/process-announce.ts

server/lib/activitypub/process/process-create.ts

server/lib/activitypub/video-comments.ts

server/lib/activitypub/videos.ts

server/lib/client-html.ts

server/lib/emailer.ts