Prominent UI button for toggling read post visibility

[CTalvio]

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** Describe the feature request below **

Essentially something like the "clear" button in the Relay reddit app. Or a filter setting.

A fast way to toggle the "show read posts" user setting would allow users to more often and easily switch between these two browsing modes. I find myself constantly flip flopping between disabling it for discoverability, and enabling it to come back to stuff.

[AaronMaria]

Possible duplicate of #598

[dessalines]

Stale issue, but if someone wants to work on this, I'll re-open.