Submission portal: require login/group, add icons. Redirection to submission portal: require: login/group. Distinctive design for organism-selector page.

[theosanderson]

Resolves #1437

• Add icons to submission portal buttons
• Require login and group for the submission portal, because it's confusing to only insist on this in the next step
• Also require these for the organism selector that will redirect to the submission portal for the same reasons
• Remove images and descriptions from organism-selector so it is less similar to the home page

https://improve-submission-ux-aro.loculus.org/

among other changes. (icons could be better sized etc., but ok for now I think)

[chaoran-chen]

Cool!