Add "Assigned" filter in addition to "Review requests"

[matklad]

would be great to see "Assigned to me". We at TigerBeetle don't use "request review" and instead always "assign" people (see https://github.com/tigerbeetle/tigerbeetle/blob/main/docs/internals/HACKING.md#pull-requests for details)

[kylecarbs]

I'll add this! Will also fix the buttons overlapping in the top-left.