com web/bugs: Refactor pull requests service class: include/classes/bug_ghpulltracker.php src/Repository/PullRequestRepository.php src/Utils/GitHub.php www/bug.php www/gh-pull-add.php

Link: <a href=";a=commitdiff;h=44706d2e93c7d334710b9f1b202005fea6f9a770" title=";a=commitdiff;h=44706d2e93c7d334710b9f1b202005fea6f9a770">;a=commitdiff;h=44706d2e93c7d334710b9f...</a>

Refactor pull requests service class

- Class moved to src under App namespace
- Code refactored into repository class and GitHub client for managing
pull requests
- When new pull request is attached to a bug report the info comment is
also added to bug activity as is practice with other activities

Changed paths:
D include/classes/bug_ghpulltracker.php
A src/Repository/PullRequestRepository.php
A src/Utils/GitHub.php
M www/bug.php
M www/gh-pull-add.php