zulip: Add horizontal scroll bars for table on overflow

Currently, if the table text overflows due to too many columns, there is no way to scroll horizontally to see the whole table. The text is rendered properly since it can be selected and copied

Here’s a markdown snippet that you can use to reproduce the issue.

dsdhdbcbbfoo|vzdvdhsdbvbar|sdvdsdhbdsbbaz|asadgdgdrsgdffefdsdf|basfsdfsdf|csadnstjdncsdsd|dszvgdgvxcvdvvsf|esadvsdvsddvgsvxv
---|---|---|---|---|---|---|---
dfbdsfbdbdb1|2sdvdrgerv|3sdvdgdrbxfd|4svvdgvxcv|5asvvvvvdv|sdvsvS|svsdvdgdr|vadsvssasvasvs|vdgdvrgr this is cut off

When pasted in a Zulip chat, it renders as below, with no way to scroll horizontally. I’m running Zulip in Firefox 78.0.2 on Linux, in case that is relevant.