[Modules] Add a text option to the item types that can be added to modules

[VPundir]

Problem statement:

Clicking + to Add an element to modules allows one to add a few types of elements: Assignments, Quiz, File, Page, Discussion, Text Header, External URL, and External Tool. However, sometimes, we need to add some descriptive text (as opposed to the text header).

Proposed solution:

Add an element type that can be added to modules: plain text.

User role(s):

instructor