navbar: Use flex-grow to align search_icon to right.
navbar: Use flex-grow to align search_icon to right.

This change allows us to align the search icon to the right end of the
navbar, without having to rely on the `margin-left: auto` trick. This
is better because it's more convenient to let flex handle the
positioning and will, hopefully, be more resilient to breakages.

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