Use black as formatter for python files

Update Custom Field logic for Netbox v2.10.x

Add function to load YAML files ...

This commit starts to remove some code redundancy from the startup
scripts for easier maintenance.

Update Rack Initializer for Netbox 2.7 ...

The rack database model has changed in Netbox 2.7.
Therefore the initializer code, that was used before, broke.
As a user, you will need to update your racks.yml file as follows:

- Rack types must match one of the 5 rack types given, e.g.
  '4-post-cabinet'.
- Rack width must match one of the 2 rack widths given, i.e. '19' or
  '23'.

See the diff of this commit for further information how this is meant.

Ignore missing initializers ...

Previously, the startup_scripts would fail if an initializer file was
not present. Now they just ignore missing files.

Prefix output messages with appropriate emoji

Increase order prefix to 3 digits