Ensure that `gitlab_standard` comes first in the list of contexts

What does this MR do?

As raised in https://gitlab.com/gitlab-data/analytics/-/issues/8099, the order of Snowplow contexts matters when we are parsing/flattening events.

To make the order consistent between frontend and backend, this MR changes the backend tracking module to always place gitlab_standard (represented in code as StandardContext) first in the list of contexts.

Based on discussion in this slack thread: https://gitlab.slack.com/archives/CL3A7GFPF/p1614977560157100?thread_ts=1614677571.084500&cid=CL3A7GFPF

Looking at this, it seems like we have an inconsistent ordering of contexts between BE and FE. On the frontend, gitlab_standard goes first:

const contexts = [STANDARD_CONTEXT];
  if (data.context) {
    contexts.push(data.context);
  } 

Whereas on the backend, gitlab_standard is added last:

context += [Tracking::StandardContext.new(project: project, user: user, namespace: namespace).to_context]

I think having gitlab_standard first makes sense, so I will make the appropriate change on the backend.

Screenshots

A good event with multiple contexts tested locally:

Does this MR meet the acceptance criteria?

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