Improve i18n pluralization for Russian

[obulat]

Description

Some languages have a more complex set of pluralization rules than 'use a plural form when a number is greater than 1'. The current i18n setup does not take this into account, and the Image Search Result page would have incorrect translation because of that.
This PR adds the pluralization rules for Russian to fix this. Even as a native speaker, my mind is blown by the complexity of the rules :)

Technical details

The pluralization function is called whenever we use $tc function of vue-i18n that chooses the correct plural noun in phrases with numbers.

Screenshots

Numbers ending with 2-4 require a different ending than the numbers ending with 5-9:

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[dhruvkb]

Learning about the Russian pluralization rules was fun!