feature: Download videos for offline use

[morgankar]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I cant view the videos offline especially in college libraries and during long train journeys as then I dont have access to wifi and have to rely on mobile data which honestly runs out very soon when watching the video halfway through.

Describe the solution you'd like
A way to save videos offline similar to youtube on desktop.
Youtube encrypts the downloaded videos and can be viewed only via the youtube website. Similar thing can be done to 100xdevs website i guess.

Describe alternatives you've considered
There are no alternatives to this to be honest.

Additional context
Please add a "download" button for each of the lectures similar to this one.

[Shivamvashist]

@hkirat @devsargam
I have made a PR for this issue.
and these are the features that i have added -

1- Videos can be downloaded by users to watch later.
2- Downloads are encrypted and can only be viewed via website only.
3- The player will also use the offline copy if its available.