improvements to the mbox import script

* ignores dot-files and empty emails
* new setting to prefer HTML over plaintext emails during import
* restore original site settings at the end of import
* elided content of HTML mails was not put inside details block

script/import_scripts/mbox/settings.yml

@@ -1,9 +1,10 @@
 # PostgreSQL mailing lists
-#data_dir: /data/import/postgres
+#data_dir: /shared/import/data
 #split_regex: "^From .*@postgresql.org.*"
 
 # ruby-talk mailing list
-data_dir: /data/import/ruby-talk/news/gmane/comp/lang/ruby
+data_dir: /shared/import/data
 split_regex: ""
 
 default_trust_level: 1
+prefer_html: false