Add "Post" feature to enable private projects

[xmatthewx]

Users will want to keep some projects private or in a draft state (particularly with hashtags and profile views unlocking discovery). Design below adds this feature.

Notes:

• new projects will be private by default
• we won't won't change privacy of existing projects
• we will add a toggle in project settings
• we will add a bold CTA in Make view (so we don't lose too many projects)

If "Name this project" CTA (#729) is also implemented, the "Post" CTA will only appear if a project has been named. If it isn't yet implemented, Post CTA should always appear on private projects.

[xmatthewx]

Design and notes by @vazquez