Fix dashboard not working when custom httpNodeRoot path is set

[Steve-Mcl]

closes #163

Description

This is a temporary* fix to get dashboard2 working when the user has set httpNodePath to move the express endpoints.

I needed to fix this to review #164 - so raised it while fresh in mind.

*temporary

There is currently no way to know (in main.js) what the base path of the Node-RED instance is but we need this to setup the socket.io.
While #23 is unresolved, this is not an issue since the dashboard is hard coded to be /dashboard so we will always be able to determine the base path by grabbing everything before /dashboard from the pages href. But when #23 is implemented, we will need a way to distinguish the following possible URLs

• http://xxxxx:port/nr/api/v1/section1/page2 // dashboard was named section1 so base URL is http://xxxxx:port/nr/api/v1
• http://xxxxx:port/nr/api/v1/section1/page2 // dashboard was named v1 so base URL is http://xxxxx:port/nr/api/
• http://xxxxx:port/nr/api/v1/page/2 // dashboard was named v1 so base URL is http://xxxxx:port/nr/api/
• http://xxxxx:port/nr/api/v1/pages/2 // dashboard was named pages so base URL is http://xxxxx:port/nr/api/v1

Related Issue(s)

#163

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on flowforge/helm to update ConfigMap Template
  • Issue/PR raised on flowforge/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• Includes a DB migration? -> add the area:migration label