Ability to add logo and color for channel

Background

A channel is often a brand with a logo and color. Adding this makes is possible to use this in (for example) themes, e-mails and in the Integrated backend.

Requirements

Pages: /admin/channel/integrated_for_developers/edit and /admin/channel/new

• Add field "Logo" --> upload field with storage object, using the integrated_image_dropzone formtype --> save logo as a storage object
• Add field "color" to choose a color with a color picker (https://itsjavi.com/bootstrap-colorpicker/ "As a component" or advice another one, load with Bower and IntegratedAssetBundle)
• Both fields are optional