Merge features from dev to master

Features:

• Allow downloading specific resolutions (e.g. HD, 4K, or numeric values like 720p) instead of the previously vaguely defined high, medium, low. Default resolution is now HD if nothing is specified. This also fixes issues caused when a lecture supplied more then 3 different resolutions.
• Added requirements.txt
• Fixed the way the date is added to the filename to be consistent between older and newer lectures. ETH changed the timestamp formatting to include seconds halfway through SS21.
• Fixed a few typos

Changes:

• Renamed a few variables
• Used some more f-strings for print formatting

Breakage:

• For the video quality high, medium, and low will no longer work. Instead use highest for highest, lowest for lowest, or a specific numeric value for the desired resolution (e.g. 720p or keywords like HD, 4K).
• As the quality of the video is also part of the filename this means what previously downloaded videos might get redownloaded when using the --all parameter. To prevent this generally it is recommended to use the --history parameter.