Add custom startup hooks

[flx5]

New Behavior

Add a folder for scripts that are run on startup. That way users can then for example create groups at startup.

Contrast to Current Behavior

Currently it is a manual process.

Changes to the Wiki

Automated provisioning:

You can run custom scripts at application startup.

For example the script 01-groups.py would create groups at startup:

from netbox.authentication import Group

for group in [  'admins', 'default-users' ]:
    Group.objects.get_or_create(name=group)

Proposed Release Note Entry

Add custom startup hooks

Double Check

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• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.

[tacerus]

Would it make sense to ship this directory (with some dummy file and the *.py in .gitignore) and to mount it in the default docker-compose setup for convenience?