v0.2.3: Merge pull request #144 from lepo-project/version-0-2-3

• Remove no-need raw helper
• Refactor pluck(:id) to _ids
• Autosize sticky text area for update
• Specify stylesheet files explicitly
• Fix export format for worksheet note
• Implement WEB_OPEN_SNIPPET_MAX_LENGTH constant
• Exclude PDF snippet from media snippet count
• Limit snippet.description.length
• Fix design for radio button group
• Fix to eliminate lesson note from default_note_id
• Change max character length for web snippet
• Record user actions in a log table
• Fix text area bug for chrome
• Use update_columns for system update
• Support for highlight to cover and assignment pages
• Fix to create content with Page model
• Fix to show pages with Page model
• Add some validations for PageFile
• Add system update preference
• Add category for PageFile