dns.cgi: Fixes bug#12395 - German umlauts not correctly displayed in remarks

dns.cgi: Fixes bug#12395 - German umlauts not correctly displayed in remarks

- If Freifunk München e.V. is entered as a remark it gets converted to
   Freifunk München e.V.
- This is because cleanhtml is used on the UTF-8 remark text before saving it to the file
   and the HTML::Entities::encode_entities command that is run on that remark text does
   not work with UTF-8 text.
- If the UTF-8 text in the remark is decoded before running through the cleanhtml command
   then the characters with diacritical marks are correctly shown.
- Have tested out the fix on a remark with a range of different characters with
   diacritical marks and all of the ones tested were displayed correctly with the fix while
   in the original form they were mangled.

Fixes: Bug#12395
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>