White label feature

HCB will be adding a while labeling feature. This is specifically white labeling the public-facing pages (NOT the HCB dashboard).

At the moment, this scope only includes the donation page, but may expand to include transparency pages in the future.

This setting should be available under an organization's settings.

To begin, we have these two tasks:

Clarify the two intended tasks and the organization-settings flow before locating the donation page implementation. The initial scope is white labeling the public-facing donation page, with transparency pages explicitly deferred; done should include an organization setting and the corresponding donation-page branding.

Written by the indexing model from the issue text.