feat: Add full customization for GitHub integration + improve task linking logic

[JustSxm]

Description

This adds a new environnement variable KANEO_DISABLE_BRANDING to disable Kaneo's branding in the github integration. In the future, this could also be used for the white labeling issue discussed in the discord #suggestion channel.|

This PR adds full customization for the GitHub integration, allowing users to define the issue title, description, and the automatic comment posted when an issue is created in GitHub and imported into kaneo.

It also fixes a bug where switching between projects (one with an integration and one without) caused the form to retain values instead of resetting properly.

Additionally, the hard-coded “kaneo” tag has been removed, as it provided no real value; in the future we could introduce customizable tags if needed.

Finally, the logic for determining whether a task already exists when receiving data from the GitHub webhook has been improved by using a new linked_issue_id column rather than relying on title.

Related Issue(s)

Fixes #628

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test addition or update
• Other (please describe):
  These new features are not a breaking change. Since we add Title Template and Description Template empty, those will use the same template that we currently use. For the Comment Template, I've created two migration. One that adds the column with the current message as a default, and another one that drops the default.

How Has This Been Tested?

• Unit tests
• Integration tests
• Manual testing
• Other (please describe):

Screenshots (if applicable)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Additional Notes