Build API handler should have no dependencies on gorm

[pwaller]

See "A motivating example" in issue #290.

It should be possible to make it so that none of the handlers in build.go reference a gorm DB directly, only through a buildRepo.

For example:

platform/handlers/api/build.go

Lines 128 to 144 in ffb20d4

	// Report fetches a build's report.
	func (b Build) Report(c *gin.Context) {
	buildRepo := models.BuildDataSource(db)
	build, err := b.ByID(c)
	if err != nil {
	return
	}
	report, err := buildRepo.GetBuildReport(build)
	if err != nil {
	sugar.NotFoundOrError(c, err)
	return
	}
	sugar.SuccessResponse(c, 200, report)
	}

Line 130 should be moved out to the main entry point, and b.ByID(c) should be broken up into

1. a thing which figures out the BuildID for the request, and
2. a buildRepo.ByID(id) call which returns the build ID.

This issue is fixed when handlers/api/build.go no longer refers to the db global variable.