Fixed #19639 - Updated contributing to reflect model choices best practices.

Thanks charettes.
Fixed #6682 -- Made shell's REPL actually execute $PYTHONSTARTUP and `~/.pythonrc.py`.

Also:

* Added a ``--no-startup`` option to disable this behavior. Previous
  logic to try to execute the code in charge of this funcionality was
  flawed (it only tried to do so if the user asked for ipython/bpython
  and they weren't found)
* Expand ``~`` in PYTHONSTARTUP value.

Thanks hekevintran at gmail dot com  for the report and initial patch.

Refs #3381.
Fixed #19577 - Added HTML escaping to admin examples.

Thanks foo@ for the report and Florian Apolloner for the review.
Merge remote-tracking branch 'upstream/master'