Implement the images style administration backend

This task focus on the implementation of the backend/api: (see XWIKI-19462 for the frontend)

• storage (XClass/XObjects)
• rest endpoint
• reusable rest client

See https://design.xwiki.org/xwiki/bin/view/Proposal/NewCKEditorimagedialog#HStyleAdministrationUI

Entry point:

• Live Data listing the existing styles + remove and edit actions (according to the user's rights).
  dropdown selection of the default style

Style edition form:

• CSS class: Free text allowing to input the css class that is attached to the images of the corresponding style. Note that the skins resources must use the same css class in their definitions. Note: Some flexibility could be possible by allowing several css classes to be defined (and in this case some skins CSS could impact several styles, allowing less code duplication)
• Position (checkbox)
• Wrap Text (checkbox)
• Border (checkbox)
• Image Size Setting:
  • Width: checkbox (Note: possibility to define a default value?)
  • Height: checkbox (Note: possibility to define a default value?)
• Image Size V/H (if style allows the default to be overridden)
  • Width: checkbox (Note: possibility to define a default value?)
  • Height: checkbox (Note: possibility to define a default value?)