Use a native OpenResty substitution filter

Problem

Currently Onionspray relies on subs_filter directives from ngx_http_substitutions_filter_module. This has some problems:

1. It prevents users from using OpenResty's upstream packages or container images.
2. It's an additional dependency besides OpenResty.
3. This codebase does not seem to be activelly maintained.

Tasks

• Use a native OpenResty substitution filter via the Lua Ngx API.

Time estimation

• Complexity: medium (3 days)
• Uncertainty: low (x1.1)
• Reference (adapted)