Refactor Instructor Tasks to be used from within the plugin

[arslanashraf7]

Related to:
#7 (comment) & #7 (comment)

Description:
We're using the instructor tasks API to run/track tasks of canvas integration and in order to do that, we kept a couple of changes in the edx-platfrom(https://github.com/mitodl/edx-platform/pull/274/files) while creating the plugin.

At that point while creating the plugins we couldn't see a direct way of migrating all of our plugin code from edx-platform to our plugin since the instructor dashboard doesn't support whole plugin-based tabs support. So, we decided to migrate the possible code sections into the plugin and decided to work on the other ones later.

What needs to be done:
As part of this ticket, we will migrate the code parts of the instructor dashboard from edx-platform(https://github.com/mitodl/edx-platform/pull/274/files) to the plugin.

This might need us to create our own API add support for instructor tasks used in https://github.com/mitodl/edx-platform/pull/274/files.