Detect if first login

[CarolineDenis]

Fixes #6211

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone

Testing instructions

• Open a blank db (with no institutions)
• Verify that you land on an empty screen displaying the message "Welcome! No institutions are available at the moment."

[grantfitzsimmons]

This code suggests that there may be multiple institution records in a single database, both in what is displayed to the user and in the API endpoint. Currently, I'm not aware of any plans to support more than one institution per database, but perhaps this decision was made with the understanding that this may not always be the case.

Is this intentional, or should it return only one institution at any given time?

[grantfitzsimmons]

Testing instructions

• Open a blank db (with no institutions)
• Verify that you land on an empty screen displaying the message "Welcome! No institutions are available at the moment."

[CarolineDenis]

@grantfitzsimmons,
updated to single institution

[sharadsw]

Code looks fine functionally. However, I think we should reconsider the route we're creating, especially after 0f25eee (sorry!)