apache config: fix office preview

[shoeper]

References:

• https://httpd.apache.org/docs/2.4/en/mod/core.html#allowencodedslashes
• https://forum.seafile.com/t/document-preview-xlsx-with-libreoffice-not-working-but-docx-pdf-md-works/9252/30?u=shoeper
• https://serverfault.com/a/143342

[shoeper]

An open question for me is whether it also needs NoDecode or just using AllowEncodedSlashes is fine. At least in the forum it was reported, that using just AllowEncodedSlashes On would be enough.