Default Account selector in Signer overlay (#4375)

* Manage default accounts

* Portal

* Portal

* Allow Portal to be used in as both top-level and popover

* modal/popover variable naming

* Move to Portal

* export Portal in ~/ui

* WIP

* Tags handle empty values

* Export AccountCard in ~/ui

* Allow ETH-only & zero display

* Use ui/Balance for balance display

* Add tests for Balance & Tags component availability

* WIP

* Default overlay display to block (not flex)

* Revert block

* WIP

* Add className, optional handlers only

* WIP

* Properly handle optional onKeyDown

* Selection updated

* Align margins

* Remove old code

* Remove debug logging

* TransitionGroup for animations

* No anim

* Cleanups

* Revert addons removal

* Fix tests

* defaultAccount

* Selection actually selects

* WIP tests

* tests WIP

* Expand tests

* Container for scrollbars

* Add parity_defaultAccount RPC (with subscription)

* Add jsonrpc interface