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).

db/upgrade/v1_to_v2.rs

@@ -33,6 +33,7 @@
 use crate::dir;
 use failure::{Error, bail, format_err};
 use libc;
+use protobuf::prelude::MessageField;
 use rusqlite::types::ToSql;
 use crate::schema::DirMeta;
 use std::fs;
@@ -113,7 +114,7 @@ pub fn run(args: &super::Args, tx: &rusqlite::Transaction) -> Result<(), Error>
     {
         meta.db_uuid.extend_from_slice(db_uuid_bytes);
         meta.dir_uuid.extend_from_slice(dir_uuid_bytes);
-        let open = meta.mut_last_complete_open();
+        let open = meta.last_complete_open.mut_message();
         open.id = open_id;
         open.uuid.extend_from_slice(&open_uuid_bytes);
     }

db/upgrade/v2_to_v3.rs

@@ -37,10 +37,11 @@ use crate::dir;
 use failure::Error;
 use libc;
 use crate::schema;
+use protobuf::prelude::MessageField;
+use rusqlite::types::ToSql;
 use std::io::{self, Write};
 use std::mem;
 use std::sync::Arc;
-use rusqlite::types::ToSql;
 use uuid::Uuid;
 
 /// Opens the sample file dir.
@@ -68,7 +69,7 @@ fn open_sample_file_dir(tx: &rusqlite::Transaction) -> Result<Arc<dir::SampleFil
     meta.db_uuid.extend_from_slice(&db_uuid.0.as_bytes()[..]);
     meta.dir_uuid.extend_from_slice(&s_uuid.0.as_bytes()[..]);
     {
-        let open = meta.mut_last_complete_open();
+        let open = meta.last_complete_open.mut_message();
         open.id = o_id as u32;
         open.uuid.extend_from_slice(&o_uuid.0.as_bytes()[..]);
     }

db/upgrade/v3_to_v4.rs

@@ -62,6 +62,9 @@ pub fn run(_args: &super::Args, tx: &rusqlite::Transaction) -> Result<(), Error>
           time_90k integer primary key,
           changes blob
         );
+
+        alter table user add column permissions blob;
+        alter table user_session add column permissions blob;
     "#)?;
     Ok(())
 }