Add base branch to prs and dashboard

[gonzaM]

Add Base Branch Filtering to Pull Request Metrics

Summary

This PR introduces the ability to filter pull request metrics by base branch, enabling more granular insights into development workflows across different branches (e.g., develop, staging).
For example, PRs targeting the staging branch (typically used for releases) are usually larger and could affect the accuracy of the overall metrics. In such cases, it may be useful to view metrics exclusively for PRs with develop as the base branch.

Changes Made

Database Changes

• Migration: Added base_branch column to events_pull_requests table to store the target branch for each PR

Backend Changes

Models

• Updated Events::PullRequest model to include base branch data
• Added base_branches method to Repository model to fetch available branches

Controllers

• Modified DevelopmentMetricsController to pass base branch parameter to metrics builders
• Updated PullRequestsController to accept and filter by base branch

Services

Enhanced multiple service classes to support base branch filtering:

• Builders::Chartkick::DevelopmentMetrics and related metric builders
• Distribution builders for pull request analysis
• Metric group services for weekly reporting

Frontend Changes

UI Components

• Added base branch dropdown filter in the metrics interface
• Updated navigation filters to include branch selection

Styling

• Added CSS styles for the new base branch filter component
• Improved spacing and layout for the filters bar

Data Processing

• Event Processing: Updated pull request event builder to capture base branch information from GitHub webhook data
• Query Optimization: Modified database queries across metric builders to conditionally filter by base branch when specified

Technical Notes

• All existing functionality remains unchanged when no base branch is selected
• The filter integrates seamlessly with existing time-based filters
• Database queries are optimized to only apply base branch filtering when a specific branch is selected

Testing

• Updated existing tests to accommodate the new parameter structure
• Ensured backward compatibility with existing metric calculations