Implement uploading lessons videos functionality

Implement the functionality to upload videos for the lessons.

Acceptance criteria:

[ ] Each lesson should contain one video.
[ ] Videos should be stored in a the local storage of the server for now until we migrate to a third party provider.
[ ] Make sure to put some limitations on the videos such as the size of each video shouldn't exceed 1GB or 500MB,
[ ] After storing the videos put there URL into the DB combining the feature from issue #28.
[ ] Make sure that the real URL of the video is returned in the get operations for lessons from the issue #28.

PandaX185 / e-learning