Send gitlab_standard context with events from the frontend

What does this MR do?

Related to #300458 (closed)

It sends the gitlab_standard as a context for all trackPageView and trackStructEvent events emitted from the frontend.

The schema URL and some of the data is hardcoded, this will be addressed on #320800 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done