* Dashboard: Remove template variables option and update style of ShareModal (#29191)

* Simplified design

* Changed to text area component for embed and replaced select with RadioButtonGroup

* Use primitive string instead of SelectableValue in the states

* Changed embed html TextArea to writable and added a copy to clipboard button

* Added some space between the buttons on the snapshot tab and removed unnessecary FieldSet elements

* Add descriptions to the tabs that were missing descriptions

* Capitalization of theme names

Co-authored-by: Torkel Ödegaard <torkel@grafana.com>