css: Fix horizontal scrolling of markdown table when overflow.
css: Fix horizontal scrolling of markdown table when overflow.

Previously Zulip's markdown tables didn't properly configure the overflow CSS rules
to allow proper scrolling.

Fixes #16205.
punchagan/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,kou/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,showell/zulip,showell/zulip,punchagan/zulip,showell/zulip,kou/zulip,kou/zulip,rht/zulip,rht/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,kou/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip