Annotation Tool: Flagging plugin should now allow instructors to remove flags from annotations

Annotation Tools: Added lots of comments, fixed spacing, fixed slice issue

Annotation Tools: i18n fixed for this tool

 - fixed Translator to Translators

Annotation Tools: Removed unnecessary class and worked with element object
Instructors can view previously sent email content

Previously on the send email page of the instructor dashboard, instructors could only view
task information about emails they've sent for their course in the past.
In addition to this, I've now added the ability to see the content of all previously sent emails.

A "Sent Email History" button has been added to the page. When clicked, a table displaying the
subject line, number of emails sent, and date/time of submission for each previously sent email
is created. An instructor can then click on any subject line to see the content of that email,
displayed in a modal window that appears on the page.

The window is also equipped with a "copy email to editor" button, which copies the emails contents
to the tinyMCE editor, so that an instructor can easily resend an email that they've sent
in the past.
Merge pull request #4378 from lduarte1991/lduarte-harvardx-pr10

Annotation Tool: Fixed Flagging for Instructor flag removal
Merge pull request #4451 from Stanford-Online/njdupoux/email-content-history

Instructors can view previously sent email content
BLD 1020: Easy access to Group Configurations page from Content Experiment component.