Refactor a SSOT for integration event descriptions

What does this MR do?

We were previously inconsistent about how we produced event descriptions for Integrations.

Sometimes we referred to IntegrationsHelper and sometimes through the model.

This change makes IntegrationsHelper the single-source-of-truth for these strings.

It also adds the TeamCity integration event description strings to our localisation files.

No tests have been added, but there are existing tests of ServiceEventEntity and the API that

Screenshots (strongly suggested)

Previously the "TeamCity integration" form was not using the strings that were defined in Integrations::Teamcity.event_description, and instead the form was displaying the "default" strings. Now, as all the logic has been moved to the helper as the SSOT, the special strings for Team City display instead of the default strings:

Editing the TeamCity integration (<project-full-path>/-/settings/services/teamcity/edit):

Before	After

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