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
adding django admin command to change enrollment
fixing example
grammar
added jj to the author list
fixed retry invokation for calling setState

to successfully call a function with parameters by setTimeout,
it needs to be a function that will be called by setTimeout internally.

this patch actually enables multiple tries for calling the setState function.
install and set up paver before using it

install and set up paver before using it
Fixes #3409
JavaScript extentions doc update

doc-255
Update screenshots and add feedback prompt info
Removed rebase error from change log
Merge pull request #3433 from edx/ahodges/documentation/changelog

Removed rebase error from change log
Merge pull request #3431 from edx/sylvia/docs/DOC-263

Update screenshots and add feedback prompt info
Added process documentation
Refer to process docs in CONTRIBUTING.rst