Bug fixes

#3020613: Center the progress throbber - Fix a CSS bug to properly center the throbber on mobile devices

- add command for demo image creation.
- split list of images into chunks for conversion or resizing with multiple batch-processes. With this modifikation its possible to handle an unlimited amount of pictures without running into "connection timed out" problemes caused by long running batch process.
- fix some styling and code cleanup

Initial commit.

Issue #3018856: Leaflet as default widget Map Library
Better "Gmap Api Key missing" messages to the users.

For branch 1.x, use webform-8.x-5.0-rc29 on Docker (https://github.com/dcycle/webform_submission_change_history/pull/1)