No time limit on unlimited access; user admin improvements

[jcushman]

• Time limit on unlimited access is now optional. Existing unlimited users keep their time limit for now.
• Update user account page to show time limits, expired time limits, or no time limits.
• Update user admin page:
  • reorganize fields
  • include research access requests
  • include search and filter options
  • allow password updates
  • drop ability to create new accounts for now, because that's tricky to support while inheriting Django form for password updates and doesn't seem important