This adds the backend pieces (no interface yet) to configure Lua scripts
that will be applied to topics and comments due to different events.
Initially, it only supports running a script when a new topic or comment
is posted. For example, here is a Lua script that would prepend a new
topic's title with "[Text] " or "[Link] " depending on its type, as well
as replace its tags with either "text" or "link":

function on_topic_post (topic)
    if (topic.is_text_type) then
        topic.title = "[Text] " .. topic.title
        topic.tags = {"text"}
    elseif (topic.is_link_type) then
        topic.title = "[Link] " .. topic.title
        topic.tags = {"link"}
    end
end

There can be a global script as well as group-specific scripts, and the
scripts are sandboxed, with limited access to data as well as being
restricted to a subset of Lua's built-in functions. The Lua sandboxing
code comes from Splash (https://github.com/scrapinghub/splash). It will
need to be modified, but this commit keeps it unmodified so that future
changes can be more easily tracked by comparing to the original state of
the file.

The sandboxing also includes some restrictions on number of instructions
and memory usage, but this might be more effectively managed on the OS
level. More research will still need to be done on security and resource
restrictions before this feature can be safely opened to users.