Add pull_requests boolean column to TaskList

[joshsmith]

Problem

We need to add a pull_requests boolean column to TaskList, similar to the inbox column, to indicate whether the column should be the recipient of new and open pull requests.

• Add the column where the default is false
• Add the column to the default_task_lists in TaskList model
  • our current "In Progress" column should be the pull_requests: true one here
• Add model tests
• Add to the views
• Add view tests
• We'll need to add migrations for existing projects to add this (can look to the priv/repo/add_default_tasks_to_projects.exs for inspiration).
  • we want the columns titled "In Progress" to be pull_request: true
  • for each project, we should find any tasks that have an open GithubIssue with a GithubPullRequest and move them to this list