Daschat-base

:label: Add or update types

* Added new schema `HandsoffNewChatSchema` in file `daschat_base/schemas.py` to be used when new chat must be created in the handsoff service.

:label: Add or update types

* Changed `path` field in `MediaMetaData` schema from `Path` to `str` type.

:recycle: Refactor code

* Create directory `schemas` to hold schemas definitions.

:label: Add or update types

* Added `slots` field to `Chat` and `Contact` schemas.
* Added schema file `slots.py` with new schamas to suppor usage of `slots` fields.

:label: Add or update types

* Added `id` field to the `MessageSchema` schema. In the future it will be used to track the origin of the message.

:building_construction:

* Added support for Python 3.7

:building_construction:

:boom: Breaking changes

:label: Add or update types

* Changed `ResultFieldSchema` schema renaming field `msg_id` to `id` and removed `text` field.

:recycle: Refactor code

* Refactored `messages.py`.