Change the project logo away from CamelCase to Pascalcase

Background

The BuildStream (CamelCase) is easily reduced to BS, carrying the wrong connotations.

I think Buildstream (Pascalcase) is much better and does not affect the branding at all, apart from the name space on Gitlab.

From the original Gitlab URL, the project itself it not actually CamelCase: https://gitlab.com/BuildStream/buildstream

Task description

• Discuss and finalise before we upload the Logo

Acceptance Criteria

Once we have closed the issue and finalised the casing to be used.

---