Text in userstuff table headers in right-to-left languages should be aligned right, not left

If you put the following HTML containing a table with Arabic text (from the translation of our "AO3 Statistics 2020: A Look Behind the Scenes" post) into a work, the headers (<th>, for both rows and columns) will be left-aligned instead of right aligned:

We need to add some CSS to basically flip the styling we use in LTR languages, similar to AO3-5737.

How to test: Add a table using an RTL language in its headers into the body of a work / an FAQ / an admin post and make sure it's displayed like this: