index.html + css: display right part of the menu with flex

This requires to reorder elements in the HTML.
Thus they are now in a 'natural' order (ie. in the same order
as displayed). We use flex to make them appear contiguously the
ones after the other. Thus we don't need anymore float: right
apart from the #menu_right_part that was added.

Fix #3805
10 jobs for feature-sc/3805-fix-upload-menu-css in 12 minutes and 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test Tools
passed #293314
test_tools

00:00:06

 
  Test Quality
passed #293315
web
code_quality

00:00:08

 
  Test Unit
passed #293316
web
test_browser_unit

00:00:08

passed #293317
web
test_server_unit

00:00:16

 
  Deploy Review
passed #293318
web
deploy_review

00:00:08

manual #293319
web allowed to fail manual
stop_deploy_review
 
  Test Functional
passed #293320
web
test_browser-functional

00:03:26

passed #293647
x86_64 docker
test_server_functional

00:06:19

failed #293321
docker x86_64
test_server_functional

00:00:06

 
  Test Functional External
passed #293322
web
test_browser-functional-external

00:01:33