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
allow editing already set results

- added setting to allow editing already set results
- added db migration for new setting
add mime type for xlsx and csv for user import

- csv and xlsx files were not being accepted by import users file
chooser
Merge pull request #58 from agramian/allow-editing-set-results

allow editing already set results
Merge pull request #57 from agramian/custom-fields-for-more-item-types

fix default value for link custom fields
Merge pull request #59 from agramian/import-users

add mime type for xlsx and csv for user import
Fix bug with new step numbers due to type casting
Merge branch 'master' of github.com:msjit/testcasedb