Feature enhancements for the WebUI plugin

[jijiechen]

Hi Jaben,

This PR contains some main enhancements onto the WebUI plugin:

• Added Headers tab to show mail headers
• Added Sections tab to show attachments and content parts in the mail message, download functionalities also provided
• Added support for inline embedded images in mail body
• Refresh the mail list automatically when user stays at the first page of the list
• Added the ability to delete all messages (Hover the 'index' button and see the 'Delete' trash button)
• Added the functionality to download raw message
• Prevent showing messages containing script/iframe tags
• Added the ability to cache static file in the client browser side
• Cleaned up some redundant JavaScript code

Please review the code, see the updated web UI at http://localhost:37408
Feedback is welcome.

[jijiechen]

Here are some screen shots:

[Jaben]

Looks great @jijiechen ! Will review this week.

[Jaben]

Looking great! @jijiechen merging