add concept of user/session permissions

(I also considered the names "capabilities" and "scopes", but I think
"permissions" is the most widely understood.)

This is increasingly necessary as the web API becomes more capable.
Among other things, it allows:

* non-administrator users who can view but not access camera passwords
  or change any state
* workers that update signal state based on cameras' built-in motion
  detection or a security system's events but don't need to view videos
* control over what can be done without authenticating

Currently session permissions are just copied from user permissions, but
you can also imagine admin sessions vs not, as a checkbox when signing
in. This would match the standard Unix workflow of using a
non-administrative session most of the time.

Relevant to my current signals work (#28) and to the addition of an
administrative API (#35, including #66).

.gitignore

@@ -7,6 +7,7 @@ cameras.sql
 node_modules
 prep.config
 settings-nvr-local.js
+db/schema.rs
 target
 ui-dist
 yarn-error.log