Redact bot key from request logs (#269)

The bot HTTP API carries the bot key as a URL path segment
(/rooms/:room_id/:bot_key/...). config.filter_parameters redacts query
and form parameters but never path segments, so the key was written
verbatim to the request log (the "Started POST ..." line) and to any
log line echoing the pagination Link header.

Add a log formatter that redacts the bot-key path segment wherever it
appears in a formatted line, and wire it into the production logger.

config/environments/production.rb

@@ -1,5 +1,6 @@
 require "active_support/core_ext/integer/time"
 require "active_support/core_ext/numeric/bytes"
+require_relative "../../lib/rails_ext/log_scrubbing_formatter"
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -50,7 +51,7 @@ Rails.application.configure do
 
   # Log to STDOUT by default
   config.logger = ActiveSupport::Logger.new(STDOUT)
-    .tap  { |logger| logger.formatter = ::Logger::Formatter.new }
+    .tap  { |logger| logger.formatter = LogScrubbingFormatter.new }
     .then { |logger| ActiveSupport::TaggedLogging.new(logger) }
 
   # Prepend all log lines with the following tags.