Russian translation (#6516)

* Import the Russian gettext translations

Co-authored-by: Nichole Haines <[email protected]>
Co-authored-by: Kate Zhevelyuk <[email protected]>

* Import the Russian vignette translations

Co-authored-by: Nichole Haines <[email protected]>
Co-authored-by: Kate Zhevelyuk <[email protected]>

* Generate the *.mo files

Unfortunately, Gettext doesn't like the %.0s workaround for %s => plural
"s" and gives a warning for different conversion specifiers.

* Manually remove the po4a leftovers

There's probably a better way to avoid translating the "this document is
also available in the following languages:" paragraph.

* Set CODEOWNERS for the new files

---------

Co-authored-by: Nichole Haines <[email protected]>
Co-authored-by: Kate Zhevelyuk <[email protected]>

Author: 3 people

CODEOWNERS

@@ -48,6 +48,8 @@
 /po/*zh_CN.po @Rdatatable/chinese
 /po/*pt_BR.po @Rdatatable/brazil
 /po/*es.po @Rdatatable/spanish
+/po/*ru.po @Rdatatable/russian
+/vignettes/ru/*.Rmd @Rdatatable/russian
 
 # printing
 /R/print.data.table.R @michaelchirico