Add cohort custom fields functionality

Basic functionality Log in as an admin Go to Site administration / Users / Accounts / Cohort custom fields Click " Add a new category " Confirm that a new category "Other fields" is added. Try to rename this category. Confirm  that you can rename the category. Refresh the page. Confirm  that the category name remains unchanged. Click " Add a new custom field " Confirm  that you see the enabled custom field type plugins on the dropdown menu. Add a custom field of Short text type (set Required and Unique as yes) Go to Site administration / Users / Accounts / Cohorts Select Add new cohort and confirm you see created custom fields under Other fields section of the form. Populate all fields, except the custom field and try to save changes. Confirm that  you can not save as the field is mandatory. Populate field with 111 and save. Confirm that a new cohort is created. Click Edit in the dropdown menu for a newly created cohort. Confirm that 111 is set for the custom field in opened edit form. Click cancel. Select Add new cohort and set category to Category 1, set all fields as you want, but the custom field as 111. Confirm that you can't save the form and you see an error "This value is already used." Change the custom field value to 222 and save the form. Confirm that a new cohort created. Click Edit in the dropdown menu for a newly created cohort. Confirm that 222 is set for the custom field in opened edit form. Web service  Should be covered by unit tests.