Fixed all links in release notes

[mhoeber]

moved links to links.rst

Doc-327

[mhoeber]

@srpearce @lamagnifica

Fixed all links in release notes; moved all URLs to links.rst for easier maintenance.

Can you randomly test links in: http://draft-release-notes-link-fix.readthedocs.org/en/latest/index.html

[srpearce]

Just a couple nits:

April 15 release notes:
Under edX Learning Management System, a link labeled "View Your Live Course" goes to "Testing Your Course"

April 3:
Link to "Custom JavaScript Display and Grading" under edX Documentation goes to Custom JavaScript Applications. (On the "Custom JavaScript Applications page, the Custom JavaScript Display and Grading link is broken.)

January 29 (these are probably fine):
Managing Discussions --> Discussions
Best practices for discussion moderators --> Guidance for Discussion Moderators
Using External Graders --> External Grader

I didn't go back farther than January 29, though I can if necessary.

[srpearce]

Otherwise, 👍