webui: Add editing attachments in user messages

[allozaur]

Close #17206

• Improved form UI component for editing user messages with options to choose model and attach files
• Allow removing/adding files to edited message
• Allow to update edited message without re-generating

Demo

demo.mp4

[ggerganov]

Can drag images in the edit box, but cannot paste image (Cmd + V) in it. Pasting images in the normal message box at the bottom of the screen works.

[allozaur]

Can drag images in the edit box, but cannot paste image (Cmd + V) in it. Pasting images in the normal message box at the bottom of the screen works.

Good catch @ggerganov! Just patched it, should be working 1:1 as the main Chat Form now :)