css: Add styling for code_external_link hyperlink.
css: Add styling for code_external_link hyperlink.

In case of previews, we tweak the positioning a bit more
to the right.

The previous styling also had the focus-within action
which isn't needed here as hovering over the codeblock
is enough to display both the icons.

rht/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,rht/zulip,kou/zulip,punchagan/zulip,zulip/zulip,showell/zulip,showell/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,rht/zulip,rht/zulip,zulip/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,andersk/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,kou/zulip,punchagan/zulip,kou/zulip,showell/zulip,kou/zulip,punchagan/zulip