Ability to add device from empty state

[Steve-Mcl]

closes #4077
closes #4227

Description

• Adds the empty state for Applications with "Add Device" Button
• Aligns the existing empty state for Instances by adding "Add Device" Button
• found and fixed issue where "Add Device" from instance did not actually assign the device to the instance (related issue Adding a device on the instance page does not assign new device to instance #4227)

Demo

Before

• "Empty State" missing from Application
• "Add Device" missing from instance
• No possibility of adding device from Application/Instance device pages
  

After

• All pages have empty State and capability to "Add Device"
  

Related Issue(s)

#4077
#4227

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 FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.64%. Comparing base (940a813) to head (708a47b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4228   +/-   ##
=======================================
  Coverage   78.64%   78.64%           
=======================================
  Files         286      286           
  Lines       13119    13119           
  Branches     2934     2934           
=======================================
  Hits        10317    10317           
  Misses       2802     2802           

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