navbar: Refactor tab_bar to nested selectors.
navbar: Refactor tab_bar to nested selectors.

This refactors tab_list styles to be under tab_bar and to use nesting.

andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,tommyip/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,kou/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,brainwane/zulip,kou/zulip,kou/zulip,shubhamdhama/zulip,punchagan/zulip,brainwane/zulip,timabbott/zulip,timabbott/zulip,tommyip/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,rishig/zulip,rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,tommyip/zulip,andersk/zulip,synicalsyntax/zulip,rishig/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,zulip/zulip,andersk/zulip,synicalsyntax/zulip,synicalsyntax/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,shubhamdhama/zulip,rishig/zulip,timabbott/zulip,timabbott/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,tommyip/zulip,brainwane/zulip,showell/zulip,rishig/zulip,rht/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,punchagan/zulip,rht/zulip,synicalsyntax/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,zulip/zulip,kou/zulip,timabbott/zulip,tommyip/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,rishig/zulip,eeshangarg/zulip