Configurable footer logos

[eilmiv]

Summary of changes

• Support space specific funder logos in footer and on about page
• Support space specific additional links in footer
• Support showing space logo instead of instance logo in footer
• Add background color to logos on spaces list to increase contrast

Motivation and context

This increases the configuration possibilities for spaces and is similar in direction to #1209 (footer logos are currently not per space configurable).

Screenshots

Funder logos on footer and about page look as before, but are now more configurable.

Logo in space list has background in primary space color which is the same used as navbar background:

... the white text would not be readable without the colored background.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.