Use different strings on exports page (#12569)

Currently the page re-uses strings from other contexts which doesn't fit
very well - strings incorrectly lowercase-d and pluralized, when they
don't need to be, because it's a table.

This commit changes page to re-use accounts.posts_tab_heading for toots,
and admin.accounts for "Following" and "Follows". This all should look
more aesthetically pleasing.