Add alternative sorting functionality for RHS view

[mickmister]

When browsing my pull requests needing review, I would like the option to sort by most recently updated, rather than just most recently created.

Here's what it currently looks like:

Please see @asaadmahmood's design in the comment below for the new version.

[asaadmahmood]

@mickmister We can have something like this:
https://www.figma.com/proto/YMvnd2wc5ibnrqIPObZjMY/Github-RHS?node-id=1%3A11&viewport=-36%2C481%2C0.5647242665290833&scaling=min-zoom

[mickmister]

LGTM, thanks for the quick response @asaadmahmood! Ticket is now up for grabs

[mickmister]

I'd like to point out that this could be applicable on all 4 categories of data shown on the LHS (open PRs, PR reviews, assignments, and notifications).