Endpoint Manager > Extension Mapping page UI issue with devices having >8 lines

When setting up any large analog gateways in the commercial Endpoint Manager the Extension Mapping page does not allow us to add lines beyond 8 without first adding it on 1-8 and then editing that extension to move it up to the line it belongs on.

Importing these devices through the bulk import system works fine, but adding one or two lines in the middle of an existing operational device can be a lot more of a pain than it should be.

When we select the brand the page is able to update the dropdowns to show only phones for that brand.  I think the reasonable thing to do here would be to do the same for the account dropdown and when I select a model have that field adjust itself to match what that device actually offers.

This is similar to the issue described in issue 17544 but applies to a different part of the UI than that user was discussing and seems to have a different limit applied.