Add support for computed columns

[gsreynolds]

• Add additional test for existing column generator functionality
• Add regex-single and regex test for hostname in incident details
• Add computed null condition tests
• Add computed columns UI
• Reduce duplication of values/ids/labels etc so they are defined in fewer places
• Clear Local Cache should really clear local storage and session storage entirely
• Ensure value is loaded from saved columns and ensure expression and expressionType are persisted as needed to set the accessor function
• Remove redundant aggregator from alert columns, always null
• Add validation to add column functions and set default state of columnType
• react-table requires id, but derive it now from value and not accessor if id is not directly set
• Column IDs are now the full value and not just the accessor
• Add e2e test for adding computed columns
• Update locales
• Remove duplicated functions
• Pass custom column as argument and construct value later. Improve e2e tests for alert and computed columns
• Guard against computed or alert column state being undefined

[martindstone]

looks great