db: Change the DB storage type to NaiveDateTime

The entire internals of the server now work using only NaiveDateTime,
since we know they are all UTC. At the fringes (LDAP, GraphQL, JWT
tokens) we convert back into UTC to make sure we have a clear API.

This allows us to be compatible with Postgres (which doesn't support
DateTime<UTC>, only NaiveDateTime).

This change is backwards compatible since in SQlite with
Sea-query/Sea-ORM, the UTC datetimes are stored without a timezone, as
simple strings. It's the same format as NaiveDateTime.

Fixes #87.

server/src/domain/sql_migrations.rs

@@ -170,7 +170,7 @@ pub async fn upgrade_to_v1(pool: &DbConnection) -> std::result::Result<(), sea_o
         struct ShortGroupDetails {
             group_id: GroupId,
             display_name: String,
-            creation_date: chrono::DateTime<chrono::Utc>,
+            creation_date: chrono::NaiveDateTime,
         }
         for result in ShortGroupDetails::find_by_statement(
             builder.build(
@@ -220,7 +220,7 @@ pub async fn upgrade_to_v1(pool: &DbConnection) -> std::result::Result<(), sea_o
         #[derive(FromQueryResult)]
         struct ShortUserDetails {
             user_id: UserId,
-            creation_date: chrono::DateTime<chrono::Utc>,
+            creation_date: chrono::NaiveDateTime,
         }
         for result in ShortUserDetails::find_by_statement(
             builder.build(