Add API endpoint for extension to query on usage

[ianb]

We want the extension to be able to check if the user needs special notification about the server going away.

We should make an endpoint at /api/check-usage which will return a JSON object with:

{
  "hasIndefiniteShots": true/false,
  "hasExpiringShots": true/false
}

Obviously this endpoint will require authorization.

[jaredhirsch]

Similar to my comment in #5260, adding an endpoint seems unnecessary. If they have authed before, we'll have a locally-stored device ID. We can just show those users a message with updates about screenshots, whether that's opening a tab to the Screenshots website on upgrade, or showing them a modal next time they try to take a screenshot.