feat(Mutual Checker, Show Originals): Combined custom dashboard tab functionality

[marcustyphoon]

Description

Resolves #1973.

Screenshots

Testing steps

[marcustyphoon]

• decide how to save selected tabs
• use radio buttons? maybe a form?
• lifecycle bug: enabling/disabling any feature resets selected tabs
• update mutual checker option name

[marcustyphoon]

I realized while working on this that if I imagine polling the entire (small) set of users that have both features enabled on purpose—or, creating a version of this UI that allows both filters to be controlled individually, shipping it, advertising it, getting a somewhat maximal (still pretty small) set of users that are interested in both features, then polling them—about whether one of the modes they want and would use regularly is "original posts from mutuals," I'm not confident that the majority of them would say no. Which seems like something I probably ought to believe if I'm working on this? :D