[Design] Make the instance logo work on light and dark backgrounds

[michaelstingl]

@tbsbdr Default instance logo (themes/owncloud/assets/logo.svg) is a white cloud with transparent background. To use it universally in desktop and mobile apps, we should make it more robust, so it can be used on dark and light backgrounds.

[tbsbdr]

Proposal

General:

• oC Logo with blue background (robust as it works on all backgrounds, ex. in Desktop Client)

Web:

• oC Logo with transparent background (works on dark & light mode in web)

[hurradieweltgehtunter]

see here: #8522
IMO we cannot alter the bright logo due to trademark restrictions. Instead we need to use a dark variant, which is added in this PR
@michaelstingl does this help you?

[kulmann]

Just to be clear, how should the logo upload feature in the admin settings behave? Should it replace all of the logos? And if you want to do fancy stuff like having one logo in the common section and another one in the web section you can only do that by manually altering the theme. Correct? @tbsbdr