Dashboards: Save tags on dashboard creation

[evictorero]

What is this feature?

When creating a dashboard with tags we weren't persisting them.
There is a flag copyTags which is false by default and we are using it to clear the tags array. When a dashboard is created, we shouldn't erase those tags.
Added a change to evaluate this flag along with isNew bool which indicates if this is a new dashboard or not.

Fixes #71387

Special notes for your reviewer:
How to test it:

Create a new dashboard

• Go to dashboard settings
• Add a tag
• Save the dashboard
• Validate the tag is being saved

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[juanicabanas]

LGTM