Prioritize reruns triggered manually on the frontend

[rpbritton]

Specifically for Charm QA, runs triggered by users manually should be prioritized over those created by the CharmUpdateMonitor.

Test Observer needs to support setting a priority on reruns.

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/TO-200.

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