Added healthcheck endpoint /status

[caronc]

Description:

Related issue (if applicable): refs #51

Added health check endpoint /status	Path	Method	Description
/status/	GET	Simply returns a server status. The server http response code is a 200 if the server is working correcty and a 417 if there was an unexpected issue. You can set the Accept header to application/json or text/plain for different response outputs.

Below is a sample of just a simple text response:

# Request a general text response
# Output will read `OK` if everything is fine, otherwise it will return
# one or more of the following separated by a comma:
#  - ATTACH_PERMISSION_ISSUE: Can not write attachments (likely a permission issue)
#  - CONFIG_PERMISSION_ISSUE: Can not write configuration (likely a permission issue)
curl -X GET http://localhost:8000/status

Below is a sample of a JSON response:

curl -X GET -H "Accept: application/json" http://localhost:8000/status

The above output may look like this:

{
  "config_lock": false,
  "status": {
    "can_write_config": true,
    "can_write_attach": true,
    "details": ["OK"]
  }
}

• The config_lock always cross references if the APPRISE_CONFIG_LOCK is enabled or not.
• The status.can_write_config defines if the configuration directory is writable or not. If the environment variable APPRISE_STATEFUL_MODE is set to disabled, this value will always read false and it will not impact the status.details
• The status.can_write_attach defines if the attachment directory is writable or not. If the environment variable APPRISE_ATTACH_SIZE or APPRISE_MAX_ATTACHMENTS is set to 0 (zero) or lower, this value will always read false and it will not impact the status.details.
• The status.details identifies the overall status. If there is more then 1 issue to report here, they will all show in this list. In a working orderly environment, this will always be set to OK and the http response type will be 200.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] Tests added