Allow posting new thread from main page

[essential-randomness]

PostEditor can now work without a default board (see #BobaBoard/boba-components#81) , but we need to pop up an error if the user tries to create a post without. Once that's done, we also need to display the "create new thread" post in the main page.

Tasks

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1. Change ContributionEditorModal to return an error if no board is selected when creating a thread
2. Add a bottom bar to the index page and wire up the new thread button
3. Allow updating which user roles are available to post as according to board selection