647 show hide read button

[joshua-davis-rose]

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• You've discussed making your changes with a member of the dev team per contributing rules in the README
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• You've tested your changes

Summary

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This addresses #647 . Details of implementation deviating from the ticket description were discussed with @ktgd .

Adds a Show Hide Read Button setting to enable/disable the visibility of the Hide Read FAB. Included a footer detailing what the Hide Read Posts on Feed and Show Hide Read Button settings do.

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Test Plan

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1. View the feed as normal. Any read posts should remain visible after refreshing the feed.
2. Enable the Hide Read Posts on Feed setting. Repeat step 1. Now any read posts should disappear on refreshing of the feed, but the Eye FAB should not appear.
3. Enable the Show Hide Read Button. Returning to the feed should now show the Eye FAB button. After reading posts, pressing the FAB should remove read posts from the feed without refresh.

[sgriff96]

I'm good with this