SWF thumbnails: support for gnash

Swfrender produced mostly black squares. Gnash handles SWF files much, much
better than swfrender.
Closed #53
Closed #52 - fixes for anonymous accounts

- Anonymous account is no longer created when commenting/uploading
- Anonymous users can now switch safety, if it's available
- Anonymous users can delete their own posts
- Refurbished session and logging in/out mechanism
- Possible fixes for registration/activation/account deletion issues
Added avatars for unknown users
Next/prev post navigation respects safety settings

Before change this setting was either ignored or errors were shown if users was
unable to view given post.