Refactor manage_plugin_page.php

• Moved duplicated code into a new function
• Remove unnecessary variable assignments and processing
• Fixed HTML indentation
• Moved misplaced <div> tag out of if statement
• Added missing </fieldset> tag
• Improve presentation
  • left-align plugin name
  • larger font size in tables
  • use <th> instead of <td> for table heading cells
  • add tooltip with plugin status on dependency color
• Add subject to plugin author mailto: URL
• Code cleanup