Add a custom field named "Github Link" to tasks and display it in the frontend.
* Add `githubLink` attribute to the `Task` model in `client/src/models/Task.js`.
* Update `createTask` and `updateTask` actions in `client/src/actions/tasks.js` to handle the `githubLink` attribute.
* Add input field for `githubLink` in `CardModal` component in `client/src/components/CardModal/CardModal.jsx`.
* Display `githubLink` as a clickable link when filled in.
* Display a button that expands into an input field when `githubLink` is not filled in.
* Include `githubLink` attribute in card data retrieval logic in `server/api/controllers/cards/show.js`.
* Include `githubLink` attribute in card data updating logic in `server/api/controllers/cards/update.js`.
* Add validation function for `githubLink` in `client/src/utils/validator.js`.
* Add unit tests for `githubLink` field in `client/src/components/CardModal/CardModal.test.js`.
* Test display of `githubLink` as a clickable link when filled in.
* Test display of button to add `githubLink` when not filled in.
* Test input field display when add `githubLink` button is clicked.
* Test `onUpdate` call with `githubLink` when save button is clicked.
* Test validation of `githubLink` format.
