* enables overwrite if dashboard allready exist in folder

* dashboard: Don't allow creating a folder named General

* dashboards: update logic for save/update dashboard

No id and uid creates a new dashboard/folder.
No id and uid, with an existing title in folder allows overwrite
  of dashboard.
Id without uid, allows update of existing dashboard/folder without
  overwrite.
Uid without id allows update of existing dashboard/folder without
  overwrite.
Id without uid, with an existing title in folder allows overwrite
  of dashboard/folder and updated will have the uid of overwritten.
Uid without id, with an existing title in folder allows overwrite
  of dashboard/folder and new will have the same uid as provided.
Trying to change an existing folder to a dashboard yields error.
Trying to change an existing dashboard to a folder yields error.

* dashboards: include folder id when confirmed to save with overwrite

* dashboards: fixes due to new url structure

Return importedUrl property in response to importing dashboards and
getting plugin dashboards and use this for redirects/links in the
frontend.