[Platform] No login dialog window shown if only one user ID exists in DB & [Menu] Change “Revert to Committed” to “Revert to Saved"

For the demo version release 8, no login dialog window will show if there is only one user ID in database.  (The login dialog shows up if there are multiple user IDs in database for users to choose from.) -- Fix issue https://github.com/nasa/mct/issues/273.

Modified the following files:
DefaultIdentityManager & TestDefaultIdentityManager

 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Code review by pull request
 Describe potential impact on other areas of the code: Startup
 Describe any test prerequisites for this fix (e.g., restart the database): None
 For bugs marked "Regression - Yes", describe the cause of the regression:
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N