Feature(cover photo): setup ground

* Setup ground for cover photo. Merged early for beta.
* Models updated to use files
* Fixed file/picture serializers
* CoverGallery component updated
* Added fileField  / pageFiles / Picture / Picture editor & tweaked crud actions to be able to post data (would need further testing)
* Fixed general info tab elements size

* TODOs are identified by WARNING

WIP #50
9 jobs for master in 3 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Check
passed #592734
docker
check_back

00:00:18

passed #592735
docker
check_front

00:01:12

 
  Test
passed #592736
docker
test_back

00:01:02

81.0%
passed #592737
docker
test_frontend

00:00:17

 
  Lint
passed #592739
docker
eslint

00:00:25

passed #592738
docker
flake8

00:00:12

 
  Svg Gen Docu
passed #592740
docker
generate_UML_svg

00:00:13

 
  Documentation
passed #592741
docker
pages

00:00:12

 
  Deploy
passed pages:deploy

00:00:00