css: Use SCSS nesting in portico-signin.scss for `.header`.
css: Use SCSS nesting in portico-signin.scss for `.header`.

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