Make imageviewer event listeners browser consistent

[catboxanon]

Description

Fixes two issues:

• The issue mentioned in the comment seems no longer applicable in the current Gradio version with Firefox, at least as far as I could tell in testing. This is removed and thus makes the picture draggable in Firefox.
• When the event listener was set to click in Chrome, and the MMB button was used, but autoscroll was enabled, it prevented the image from being opened in a new tab. This uses mousedown in all browsers so not to interfere with autoscroll.

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests