Add the `dev-reviewed` keyword to Trac

[desrosj]

When a release has entered the RC stage, 2 committers are required to sign off on a change before it is allowed to be backported. The progress of the reviews is tracked by using keywords:

• The first reviewer should set the keywords commit dev-feedback.
• The second reviewer should set the keywords commit dev-reviewed.

The dev-reviewed keyword is not currently available in the list of keywords and must be manually entered. To make it easier, this keyword should just be added to the dropdown and restricted to only allow privileged users to apply it.

More information on the keyword can be found in the ​Handbook.

[dd32]

In 10509:

Trac: Add dev-reviewed to the trac keyword dropdown list for privledged users.

Props desrosj.
Fixes #5541.