Is your feature request related to a problem? Please describe the problem you're trying to solve.

While 4.0 is probably a better license, lots of media still is under "only" the 3.0 license. The result of checking on the official plain-text version of this license:

License:        NOASSERTION
Matched files:  MEDIA_LICENSE.txt
MEDIA_LICENSE.txt:
  Content hash:  8d8899176a20aeadf20c80f9a6140ba54b50fcb0
  License:       NOASSERTION
  Closest non-matching licenses:
    CC-BY-SA-4.0 similarity:  34.54%
    GFDL-1.3 similarity:      32.98%
    ODbL-1.0 similarity:      32.60%

Describe the solution you'd like

I'd like support for cc-by-sa-3.0 support.

Describe alternatives you've considered

Additional context

The official licence text (in plain form): https://creativecommons.org/licenses/by-sa/3.0/legalcode.txt

An example of a community with tons of cc-by-sa-3.0 media is the Minetest community.