Enable configuration of site logo via UI

[magnified103]

Description

Type of change: new feature

What

Site logos can now be configured directly in UI, through misc_config. User-uploaded static files are served under the /static-upload URL prefix.

This PR also imposes unique constraint on the MiscConfig's key field.

How Has This Been Tested?

Tested locally.

Checklist

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• I have explained the purpose of this PR.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the README/documentation
• Any dependent changes have been merged and published in downstream modules
• Informed of breaking changes, testing and migrations (if applicable).
• Attached screenshots (if applicable).

By submitting this pull request, I confirm that my contribution is made under the terms of the AGPL-3.0 License.