Add support for multiple simultaneous database features by using macros.

Diesel requires the following changes:
- Separate connection and pool types per connection, the generate_connections! macro generates an enum with a variant per db type
- Separate migrations and schemas, these were always imported as one type depending on db feature, now they are all imported under different module names
- Separate model objects per connection, the db_object! macro generates one object for each connection with the diesel macros, a generic object, and methods to convert between the connection-specific and the generic ones
- Separate connection queries, the db_run! macro allows writing only one that gets compiled for all databases or multiple ones

src/error.rs

@@ -34,6 +34,7 @@ macro_rules! make_error {
 }
 
 use diesel::result::Error as DieselErr;
+use diesel::r2d2::PoolError as R2d2Err;
 use handlebars::RenderError as HbErr;
 use jsonwebtoken::errors::Error as JWTErr;
 use regex::Error as RegexErr;
@@ -66,6 +67,7 @@ make_error! {
     // Used for special return values, like 2FA errors
     JsonError(Value):     _no_source,  _serialize,
     DbError(DieselErr):   _has_source, _api_error,
+    R2d2Error(R2d2Err):   _has_source, _api_error,
     U2fError(U2fErr):     _has_source, _api_error,
     SerdeError(SerdeErr): _has_source, _api_error,
     JWTError(JWTErr):     _has_source, _api_error,