pullrequests: refactor PullRequestModel().create The database lookups of the user and repositories are moved outside, into the callers, which have already looked up the relevant objects, thus saving two database lookups when creating a PR.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.