BUG #15347: Unaccent for greek characters does not work

The following bug has been logged on the website:

Bug reference:      15347
Logged by:          Tasos Maschalidis
Email address:      tas.o.s@hotmail.com
PostgreSQL version: 9.3.18
Operating system:   Ubuntu 4.8.4
Description:

Call to unaccent function with greek characters does not return the greek
characters without the accents as expected (not even just the few diacritics
used in modern Greek). While the customization of unaccent.rules is an
option for dedicated servers, most cloud services do not provide write
access to the file system and thus this is limiting the unaccent feature for
greek characters. This forces us to find workarounds for something
relatively simple (just some extra characters with diacritics in the
official dictionary). Please find more details in this answer of Stack
Overflow: https://stackoverflow.com/a/49849260/5909738

Thank you,
Tasos Maschalidis