A mouse's back button should work as a Zulip back button

[klardotsh]

This FR stems from a CZO discussion on this issue.

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Problem Description

I tend to use Zulip in Firefox/qutebrowser instead of the Electron app, in large part because my trackball's back button (MOUSE4 I'm pretty sure) works as a back button to navigate history there. This does not work in the Electron app, breaking my muscle memory. We have "back" functionality in the form of the toolbar icon in the bottom left, but it's not bound to the mouse.

Proposed Solution

The simplest solution is likely to attach a button press handler for MOUSE4 to the back button functionality found in the toolbar.

I haven't entirely thought through what a good UX for trackpad users would be: typically on (particularly Apple) trackpads, swiping in from the left serves as a back button, which most browsers demonstrate in some animated form by either painting a visual snapshot of the prior page, or by drawing an arrow overlay, to avoid a sudden and "invisible" back-button behavior.

Describe alternatives you've considered

None yet.

Additional context

Tangentially related to #1269.

[caraballofelix88]

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[schewara]

also related to this CZO Topic