mgr/dashboard: Cleanup

mgr/dashboard: Cleanup

- Make a method static instead of keeping it as classmethod
- Fix return type of methods type hint
- Reformat to fit 100 chars max width
- Rename some variables, methods and argument names for a better
  understanding

  Examples:
    - activatedCompression() -> hasCompressionEnabled()
    - extendByItemsForSubmit() -> assignFormFields()
        - api -> fieldName
        - name -> formControlName
        - fn -> replaceFn
        - edit -> editable
- Extract type declaration to interface and adapted the code to use
  that interface
- Replace unused nearly-global variables with local ones
- Add types where missing and appropriate
  Exmaples:
    - PoolService::getInfo() -> Observable<PoolFormInfo>
- Moved model declaration to shared directory
- Added documentation for a method which isn't that easy to understand

Signed-off-by: Patrick Nawracay <pnawracay@suse.com>