Data import settings page's 3rd pane for File Extensions: Save the new file extension

Purpose: To be able to create new settings for handling various types of file extensions. This setting will allow a user to indicate whether an extension represents a particular type of file or files, and whether a file type should be excluded from data import.

As a staff person
I want to be able to save the file extensions that I have created (finishes the creation story UIDATIMP-56)
So that they appear in the list of acceptable/unacceptable file extensions

Live prototype:
https://xd.adobe.com/view/0bb2d8ab-aabd-4ff6-4d5a-faa59a93368e-7d83/

Brief video walk-through
https://www.dropbox.com/s/3slcv4v49qm92rv/2018-11-06-data-import-19.mp4?dl=0 (see 4:10-5:10 in the video)

Scenarios

1. Scenario 1 A-M: I'm getting the unsaved changes modal, but the file extension is actually saved. Will create a separate bug for it. (UIDATIMP-155)
   • Given the New file extensions screen
   • If a User has filled in all required fields and presses the "Create" button
   • Then the record should be saved
   • And the details screen closed
   • And the user returned to the File extensions list (with the new file extension in the list), and with the Fourth pane details showing for the newly-created file extension, as per attached 05-Settings File extensions File extension detailsRev1.png
2. Scenario 2
   • Given the attempt to save
   • If the file extension is valid (starts with a period, no problem characters like slashes or spaces)
   • Then save the record
3. Scenario 3
   • Given the attempt to save
   • If the file extension is not valid
   • Then do not save the new file extension
   • And display a red "Please enter a valid file extension" message under the file extension field
   • And keep the user to the editing screen to correct the data
4. Scenario 4 See screenshot, attached
   • Given the attempt to save
   • If the new file extension duplicates an existing file extension
   • Then do not save the record
   • And display a red unsuccessful toast: New record not created: File extension [file extension] already exists
5. Scenario 5 See screenshot, attached
   • Given the attempt to save
   • If there is a communication issue with the backend, and the save is not successful
   • Then display a red unsuccessful toast: New record not created: Communication problem with server. Please try again