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.

config/locales/en.yml

@@ -719,7 +719,6 @@ en:
     blocks: You block
     csv: CSV
     domain_blocks: Domain blocks
-    follows: You follow
     lists: Lists
     mutes: You mute
     storage: Media storage