Implement Knowledge Base Contributor Markdown Upload Page

[yizou275]

This issue implements the "upload" and "reupload" page ("reupload" happens when a contributor resubmits a guide after requested change). This includes creating backend views and forms that save markdown file into db objects, and creating frontend templates that make the site appear according to figma prototype in #508.

[yizou275]

Did backend work in knowledge-base/backend-contributor branch. Will update after reorganizing the branches.

[yizou275]

Ok I now update my code in knowledge-base/contributor/upload-mdfile-backend branch, with all the backend features. Will double check tonight and make a pr for this soon.