Add CiviLink model and Graph API

[reedjones]

start at closing #149

#1438 (comment)
added a basic graph template at threads/templates/graph.html
graph visual is still kinda wonky, need to fix it but the basic foundation is there

Description:

This PR introduces a graph-based API and utilities for the application, including:

Graph Data API:

Endpoints for analyzing relationships between entities using networkx.
Get the most caused problem (/api/graph/most-caused-problem).
Get the most effective solution (/api/graph/most-effective-solution).
Retrieve the shortest path between a problem and a solution (/api/graph/shortest-path/<problem_id>/<solution_id>).
Export graph data in a Cytoscape-compatible format (/api/graph/data).
Management Command for Dummy Data:

A new Django management command, load_graph_dummy_data, creates dummy Civi and CiviLink objects for testing graph functionalities.
Includes predefined dummy data with relationships like response, rebuttal, and support.
Graph Utilities:

Added graphs.py with utility functions to:
Load graph data from the database.
Analyze graph metrics like the most caused problems, most effective solutions, and shortest paths.
Model Enhancements:

Extended the Civi and CiviLink models to support graph-related functionality.
Civi model now includes methods for exporting graph-compatible data (node, weight, etc.).
CiviLink model supports edge representation in graphs with a relation_type field.
URL Configuration:

Updated URL patterns to include the new graph_api endpoints.
Highlights:

Added CiviLink model to represent directed relationships between Civi objects.
New API endpoints powered by networkx enable advanced graph-based analysis and insights.
Dummy data loader ensures quick setup for development and testing.

[pep8speaks]

Hello @reedjones! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file project/core/management/commands/load_graph_dummy_data.py:

Line 31:121: E501 line too long (134 > 120 characters)
Line 40:121: E501 line too long (124 > 120 characters)

• In the file project/threads/graph_api.py:

Line 54:60: E231 missing whitespace after ':'
Line 60:1: E303 too many blank lines (4)
Line 87:54: W292 no newline at end of file

• In the file project/threads/graphs.py:

Line 5:34: E231 missing whitespace after ':'
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• In the file project/threads/models.py:

Line 273:20: E231 missing whitespace after ':'
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Line 467:121: E501 line too long (121 > 120 characters)
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