Adds legendText prop to timeseries to permit internationalization

What does this MR do?

Adds the legendText prop to the Time Series component. This is to enable internationalization of the abbreviations Avg and Max. The prop will be passed down all the way to the component in Gitlab-UI, which in turn will pass it to the Legend component.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Part of: gitlab-ui#245 (closed)