Add ability to clone a dashboard

[russorat]

It would be useful if a user could clone and edit an existing dashboard in order to get started. This would be probably be most useful in the pre-defined dashboards from the telegraf agents (but i'm not sure that's possible).

Once we have multi user support, users will most likely want to share their dashboards with other users. A user might not have permissions to (or want to) edit the shared dashboard, but they should be able to clone it and edit their own copy. This would be useful in orgs where new users are joining and don't want to build a new dashboard from scratch.

[avimar]

There's a lot of pre-defined dashboard elements from telegraf that would be great to be able to copy into a dashboard that I'm working on.

[russorat]

#3103