Add external link in Abiquo header menu

Author: @Xavier Thevenot

To enable users to easily open an external link to a user management endpoint, configure Abiquo to display an additional icon in the main menu. 

Abiquo will only display the external link when it is configured and the users have the appropriate privilege.

To display the new icon do these steps:

• In client-config-custom.json file, configure the "client.header.link.usersmanagement" client property

External URL

• For all users who will work with user management, check that their roles have the "Manage users" privilege (in the API USERS_MANAGE_USERS). The default cloud admin and enterprise admin roles have this privilege.

The default label for the icon is ext_link.  To modify this label, edit the lang_en_US_custom.json file, and set the value of the mainmenu.button.usersmanagement label:

Override the label