Adds support for github discussions (issues and pull requests).

With this change, a toggl button is added to github discussions (issues and pull requests) that uses the issue number and discussion title as the time entry description. The button is in the "infobar" (the bar right under the title with controls such as assigning users) and is styled as other github buttons are.

Things I'm not entirely sure about:
* Should the button only be shown on some subset of repos, such as those that the user is watching, or has starred?
* Should the discussion number be prepended to the task description? I like it because it adds context for issues that are not phrased as tasks.