no issue
- Cache invalidation header was set wrongly in frontend theme service
- This moves cache invalidation out of theme service to themes controller by passing `themeOverriden` flag along with theme
refs #10790
- Following TODO in theme index file was waiting for 2 years, and today is the day to cross it out:
- "Reduced the amount of things we expose to the outside world"
- "Made this a nice clean sensible API we can all understand!" - by @ErisDS
- Cleaned exposed methods from themes module
- Removed unused storage getter
- Removed list method
- Removed validate method
- Renamed Storage to ThemeStorage
- Named the file the same way the class defined inside of it is named
- Naming was conflicting with coming rename of `settings` -> `storage`
- Renamed theme settings to storage
