Add SHOW_PROGRESS_SUCCESS_BUTTON feature

Show a button at the top of lms/templates/courseware/progress.html if the
lowest nonzero grade cutoff has been reached. Introduce two settings:
PROGRESS_SUCCESS_BUTTON_URL is the href of that button (the course id is
appended); PROGRESS_SUCCESS_BUTTON_TEXT is the text, defaults to
"Download your certificate"
Make the SHOW_PROGRESS_SUCCESS_BUTTON feature more flexible

The button text default is now in the template and i18n-able, a
PROGRESS_SUCCESS_BUTTON_TEXT_OVERRIDE option is offered in settings.
The button url now uses string formatting.
Add test in import code to make sure two overlapping courses aren't imported
Refactored terminology and added loop break for optimization
Switch PDF Textbooks to use portable URLs
install and set up paver before using it

install and set up paver before using it
install and set up paver before using it

install and set up paver before using it
Added process documentation
Refer to process docs in CONTRIBUTING.rst
Include analytics team in dev process
link to docs to rebase a pull request
Added a visualization of the PR process
core committers, not contributors
Add example for how scrum teams spend time reviewing contributor pull requests
remove duplicate "should"
Wait that all resources are loaded and fragments rendered before initializing the xblock
Changes for batch add beta testers feature
Add information about per-criterion feedback
Merge pull request #3390 from edx/ahodges/documentation/LMS2476

Ahodges/documentation/lms2476
Removed evil HEAD line