navbar: Use direct child selector to target spans.
navbar: Use direct child selector to target spans.

The navbar uses rendered markdown and rendered html within the narrow
description, this inserts eg katex--html and allows rendering of
inline math formulae. Unfortunately, in the previous SCSS file, this
fact was overlooked and a generic "span" selector was used with would
target all spans within the parent element, direct descendants or
otherwise, which caused the side effect of applying padding and margin
to inner katex elements which broke appearance.

This commit replaces the "span" selector with "& > span" so that only
spans which are the direct children to the parent element are selected
and katex--html is rendered correctly.

Fixes: #14947.

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