server: refactor + add type hinting

- Added type hinting (for now, 3.5-compatible)
- Split `db` namespace into `db` module and `model` namespace
- Changed elastic search to be created lazily for each operation
- Changed to class based approach in entity serialization to allow
  stronger typing
- Removed `required` argument from `context.get_*` family of functions;
  now it's implied if `default` argument is omitted
- Changed `unalias_dict` implementation to use less magic inputs

server/mypy.ini

@@ -0,0 +1,14 @@
+[mypy]
+ignore_missing_imports = True
+follow_imports = skip
+disallow_untyped_calls = True
+disallow_untyped_defs = True
+check_untyped_defs = True
+disallow_subclassing_any = False
+warn_redundant_casts = True
+warn_unused_ignores = True
+strict_optional = True
+strict_boolean = False
+
+[mypy-szurubooru.tests.*]
+ignore_errors=True