Provide ability to display timestamps in different formats.

## What?

As a Graylog operator, I'd like the ability to change the ways in which timestamps are displayed so that they're more readable, in this case, using a 12 hour format.

## Why?

See HS-604750109 for details. In addition to the ticket details, it would be ideal if we could support multiple date/time compliant with ISO 8601. While the issue mentioned deals with providing the ability to display timestamps using a 12 hour format, there are also cases where supporting a different date format would also useful (e.g., EU displays of dates differ from US date displays).