Add info about template for Checkbox problem type

Change text to "click Checkboxes on the Common Problem Types tab" and
add image of checkbox button
Correct minor formatting error
Add JSInput template information

Update Advanced Problems page with Custom JavaScript Display and
Grading (JS Input) problem info. Delete separate page for JS Input
problems. Update Appendix F with new code for sample JS Input problem.
Update JS Input to reflect new template in Studio

Update Advanced Problems file and Appendix F
Update JSInput info with feedback from Jean-Michel

Add more specific info about HTML+CSS+JS files, SOP
Update index

Remove javascript_input and create_lti; add tools
Merge pull request #2023 from edx/sylvia/documentation/BLD-619

Add info about template for Checkbox problem type