Fix duplicate headers being sent by nginx

[codyro]

The only noteworthy change is I opted to send X-Frame-Options: DENY instead of X-Frame-Options: SAMEORIGIN as it's more restrictive, and I don't believe Lemmy needs anything more lax. It is also in line with the documentation instructions from install from scratch, which uses the nginx.conf from this repository, which uses X-Frame-Options: DENY.

Before

vary: Accept-Encoding
vary: Accept-Encoding
x-content-type-options: nosniff
x-content-type-options: nosniff
x-xss-protection: 1; mode=block
x-xss-protection: 1; mode=block
x-frame-options: SAMEORIGIN
x-frame-options: DENY

After

vary: Accept-Encoding
x-content-type-options: nosniff
x-xss-protection: 1; mode=block
x-frame-options: DENY

• Internal/external nginx were sending duplicate headers, removed from internal
• Opted X-Frame-Options DENY as default
• Fixes #143