Implement Subjection creation flow

[JeroenDeDauw]

AC:

• A simple temporary UI element that allows starting the "create subject flow" is shown on existing pages. This does not need to look nice. It opens the (to be created) "Creation Choice Dialog"
• The "Creation Choice Dialog" allows picking an existing Schema or choosing to create a new Schema. The former opens the Subject Editor, the later opens the Schema Editor, in such a way that when the new Schema is created, the Subject Editor is opened
• On exiting the flows, AutomaticInfobox should is shown by inserting the temporary HTML content thing we have into the page

Key considerations:

• Our Figma mockups are only directionally correct; they still have a bunch of issues.
• As usual, not everything needs to be implemented at once. The "create new schema" flow in particular seems like most of the work for a small part of the value, so should not block the "use existing schema" flow. Inserting the AutomaticInfobox HTML thing is also a nice dedicated task

Figma

• Mockups
• Prototype

Related chats:

• https://chat.professional.wiki/pro-wiki/pl/uofyzgcehfgcxbxadsgkqws6ea (UI)
• https://chat.professional.wiki/pro-wiki/pl/ss438pj18pd3feoh3zauhfzc9w (related but seperate add-infobox flow, out of scope here)

Existing code:

• SubjectEditor(Dialog).vue
• SchemaEditor(Dialog).vue
• SchemaStore (for getting Schema names (old usage), new implementation might differ, i.e. maybe we just list all Schema for now rather than doing search autocomplete)
• AFAIK current master does not have anything for the Creation Choice Dialog or button yet, though we removed some old shitty code that had some of it. My level of confidence here is not high, there might be stuff I have forgotten about

[JeroenDeDauw]

Coda inspiration for "Creation Choice Dialog":

Our Figma mockup:

Better to use "Create new" or "Create schema" instead of "Create blank". Also best to omit the Subject label, as we don't have it yet at this point, and the user should be able to provide it on Subject creation in the Subject editor.