Bug: Creating a PR does not abide by an existing pull request template file

[RCheesley]

When Druks creates a PR via Software Factory, currently it doesn't abide by the repository's existing PR template. This would be a problem for open source projects who have strict requirements in that regard, for example.

Druks should look to the repo for the template (in .github/PULL_REQUEST_TEMPLATE.md) and ensure that its description when raising a PR is rendered in the appropriate format.

It should also write in the description 'fixes #' so that GitHub (and other tools, I guess) connect it up with the issue.

[czpython]

Thanks for the report. #542 makes Software Factory fill in the repo's pull request template when it opens a PR. The plan goes inside the template. Later revisions keep the fields a person filled in.

We will not add Fixes #<number> for now. Software Factory takes its tickets from Linear, Jira, or the Druks board, not from GitHub issues, so it has no GitHub issue number to write. For Linear tickets, a closing word would also start Linear's own merge automation. Druks already moves the ticket when the PR merges, so the two would conflict.