Demo mode

• Create a "Demo bank" (demo-fake-bank) access that does not throw errors and without duplicates
• Rename current "Fake bank x" access in "Debug fake bank x" (dev-fake-bank)
• Store the mode in the default settings (default: production)
• Create a screen explaining what the demo mode will do and which creates automatically a "demo-fake-bank" access with default categories when submitted
• Add a banner in demo mode with a button to go back to the init screen, which also removes all current accesses/categories/budgets…
• Do not show the fake banks in production mode in the banks list
• Add a config entry to set a "dev" mode and only display the dev fake banks if this mode is enabled