* POC page_load_context header
* refactor to make this.pageLoadContext a private field, and relocate setTimeout code
* add pageLoadContext to `ObserveAPIEndpointDuration` method
* move to telemetry_actions
* move PageContext constant in webapp
* give server control of possible page_load_context values
* variable name change
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Co-authored-by: mickmister <mickmister>
Co-authored-by: Mattermost Build <build@mattermost.com>