Add missing HEADER_MESSAGE_ROOT key, updated some English to Greek

[nikspyratos]

@lcharette Kindly pointed out that there was still a missing locale key, so I added that.

Also modified two other keys to be less Greek-lish.

Let me know if there's anything missing.

[codecov]

Codecov Report

Merging #1042 into hotfix will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             hotfix    #1042   +/-   ##
=========================================
  Coverage     66.66%   66.66%           
  Complexity     1931     1931           
=========================================
  Files           162      162           
  Lines          6737     6737           
=========================================
  Hits           4491     4491           
  Misses         2246     2246

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[amosfolz]

Is there anything more appropriate for captcha, email, or slug?
https://github.com/nikolaos-spyratos/UserFrosting/blob/feature/greek-locale-additions/app/sprinkles/core/locale/el/messages.php#L15

[nikspyratos]

On my old branch where I did my own version of the translations I also couldn't find a good translation for slug. In Greek, slug is translated as "naked snail" and the language isn't always as malleable with using words for new meanings.
email is colloquially used in Greece instead of the transliteration (which basically means "electronic post").
captcha is more of a pronoun on its own and Greek maintains foreign terms like that.

[lcharette]

FYI, for the French locale I used the equivalent of "token" instead of slug. Because the only translation in French would also be "naked snail" :P

[nikspyratos]

FYI, for the French locale I used the equivalent of "token" instead of slug. Because the only translation in French would also be "naked snail" :P

Great idea actually. Updated using "token" for slug. "Τεκμήριο" fits well enough.

[lcharette]

Merged ! Thanks!

Ref #1027