Sanetize 'target' attr fix

[bartbutenaers]

Description

The sanetization of injected html into the ui-text node (see other PR) seems to remove a bit too much attributes, because the target attribute of hyperlinks is also removed.

The DOMPurify library offers two ways to customize which attributes should be allowed:

• ALLOWED_ATTR can be used to replace the entire list of default allowed attributes. Which imho seems to be a bit too invasive.
• ADD_ATTR can be used to retain the default list of allowed attributes and to add a few more.

I have used the latter option to add allow the link attribute. Of course extra attributes can be added in the future if necessary.

The result:

Related Issue(s)

Closes #1464

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

[joepavitt]

This conflict should get resolved once #1463 is merged, so will wait for Steve's eyes on it before moving forward with this one.