allow editing already set results

- added setting to allow editing already set results
- added db migration for new setting
fix default value for link custom fields

- new link type custom fields require two comma separated values for
the link and display name
- previously if no value was set for the field it would display a link
to the current testcasedb item’s page
- added fix to leave entry blank if the value is nothing
add mime type for xlsx and csv for user import

- csv and xlsx files were not being accepted by import users file
chooser