Use protocol relative link for training missions

This change means that on HTTPS pages, we will use a HTTPS IFRAME URL
for the Oppia instance.

Refs #1348
Merge pull request #1405 from paulproteus/end-mixed-content

Use protocol relative link for training missions
Adds doc on issue tracking and fixes small indent error
Merge pull request #1406 from willingc/doc-issue-tracker

Adds doc on issue tracking and fixes small indent error #1404
Remove un-used column distinct_months from Citations model

Includes south migration.
Remove an un-used citations test fixture.
Merge pull request #1407 from onceuponatimeforever/remove_column

Remove un-used column distinct_months from Citations model