Migrate adding and editing FEGs gateways to new style

[thmsschmitt]

The design of the dialog to add and edit FEG gateways is outdated and does not match with the design that is used for LTE gateways.

FEG

LTE

There is an extra gateway page for FEG that also uses an outdated design and that has the same functionality as the equipment page. This page does not exist for LTE networks. The only extra functionality that the FEG gateway page provides is the "Add Gateway" button.

Requirements:

• The "Add Gateway" should be moved to the equipment page
• The equipment should become the second page in the sidebar (like it is for LTE)
• The gateway page should be removed
• The FEGGatewayDialog should be styled to look like the corresponding dialog for LTE