Custom Header should reuse existing media frame

[duck_]

Unlike custom-background.js, custom-header.js doesn't attempt to reuse an existing media frame. See [22502] and [22505].

This causes strange behaviour when closing and re-opening the media modal, e.g.

1. Click "Choose Image"
2. Close the modal
3. Click "Choose Image"
4. Attempt to go to the "Upload Files" tab

Expected: go to upload files

Actual: file browser opens because a new uploader instance was created with no left/bottom positioning CSS and blocks the link text

(NB: this missing CSS when multiple uploaders are created sounds like a potential issue elsewhere too)

Potentially the cause of #22460

In 22791:

Media: Refresh the window-wide uploader whenever a state is activated. Properly unbind controller events. fixes #22504, see #21390.