DEV: enable frozen string literal on all files

This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.

Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging

lib/plain_text_to_markdown.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class PlainTextToMarkdown
   SIGNATURE_SEPARATOR ||= "-- ".freeze
 
@@ -15,7 +17,7 @@ class PlainTextToMarkdown
     prepare_lines
     classify_lines
 
-    markdown = ""
+    markdown = +""
     last_quote_level = 0
     last_line_blank = false
 
@@ -172,7 +174,7 @@ class PlainTextToMarkdown
   end
 
   def escape_special_characters(text)
-    escaped_text = ""
+    escaped_text = +""
 
     text.split(URL_REGEX).each do |text_part|
       if text_part =~ URL_REGEX