Move STT and image generation to the assistant UI

[julien-nc]

• Fix image generation result page for non-authenticated users
• Add an STT task type in the assistant UI and implement a specific input form
• Add an image generation task type in the assistant UI and implement a specific output field
• Support STT and image generation in the assistant task result page
• Use the assistant result page as target for STT and image generation notifications
• Allow to schedule (instead of runOrSchedule) image generation tasks (when canceling a sync task with the "run in the background" button)
• Harmonize image generation workflow with the rest: don't show the result widget if it has been scheduled, set notifyReady to true automatically
• Use @nextcloud/moment instead of humanize-duration for time estimation calculations and human readable display
• Downgrade @nextcloud/moment to avoid compilation warning
• Set max height of NcRichContenteditable to 300px, fix top/bottom borders not visible in some cases
• Set background color of modal close button to make it more visible when it overlaps with content while scrolling

Open questions:

• In the image gen and STT pickers, should we just use the openAssistantForm function?

[julien-nc]

This will probably get more polishing and fixes in a future PR.