Polish new/edit offering and new/edit studio pages

[rjt-rockx]

• increase max width for page contents to match the rest of the app
• move image upload areas into card previews wherever applicable
• respect image aspect ratios on upload areas too
• expand default option section by default
• break up contents into steps on new offering page
• allow specifying social links one by one rather than all at once
• review copy, icons, placeholders and overall styling