UI: Display information about what triggered a build

[flosell]

We could add more information on what triggered a build, e.g. display something like this in the list of builds:

• build #42 triggered from git commit by John Doe
• build #43 triggered manually

The information would be supplied by metadata following a particular convention.

Related:

• Introduce the concept of build-metadata #138 and Introduce a concept of step-metadata #86 introduce metadata for builds and steps
• UI: Display information about sign-offs #160 would extend this to sign-offs
• Use build/step-metadata lambdacd-git#20 uses metadata