Replace story in science with narrative xblock.

[symbolist]

Renames Story in Science to Narrative.

Testing instructions:

Checkout branch in /src/

make studio-shell
pip install -e /edx/src/labxchange-xblocks/
exit
make restart-studio

make lms-shell
pip install -e /edx/src/labxchange-xblocks/
make restart lms

• In http://localhost:18010/settings/advanced/course-v1:edX+DemoX+Demo_Course, add lx_narrative to "Advanced Module List".
• Check that you are now able to add Narrative XBlocks to units.
• After publishing the XBlock renders correctly in LMS as well.

[symbolist]

If this isn't intended to be viewed through the lms (but rather solely on labxchange), then that's fine, ignore this.

@swalladge Yup, to avoid the need to upstream changes whenever we need to update styling, we are keeping all the styles in the LX frontend.