Signatures plugin: Add settings to plugin.

Additional settings:
Max number of images
Max signature length
Max height of images
Signature Plugin: Settings updates

Ensure signature settings play nicely with ranks. 
Separate site signature settings and user signature in view.
Update the signatures icon.
Tweak signature rules.

- Enforce the max image height in a style tag, not image attributes.
- Change the way the signature rules help is built.
Only show the signature rules string if the user has permission to edit signatures.
Merge pull request #151 from vanilla/feature/expand-signature-settings

Signatures plugin: Add settings to plugin.