Stop the dark-header logo filter from blanking custom logos

[amjadr360]

Some site logos were showing up as a blank white block. Two of the newer headers sit on top of the hero image and turn the logo white so it can be seen against the dark photo. That works for a simple one-colour logo, but a logo that carries its own background just becomes a solid white rectangle.

Now only the logos we generate get the white treatment. Anything the site owner uploaded keeps its own colours and sits on a light rounded backing so it still reads against the dark header.

Both of those headers also stop showing the site name next to the logo. An uploaded logo nearly always has the name in it already, so it was appearing twice.

• One trade-off: a logo that's already white or very pale will now sit on a light backing and be hard to see. Much rarer than the case this fixes, and solving it properly means inspecting the image itself, which is a follow-up.
• Dropping the site name only affects sites built from here on. Existing sites keep the header they already have, so they'll still show it.

[kossmann]

@KevinBatdorf this is working as expected (test here)