feat: adjust button styling to match the ones from the LMS

[Agrendalath]

Description

Note: Please read the code commit-by-commit.

This:

1. Changes the structure of the "Submit" and "View results" buttons to reuse the styling from LMS.
2. Adds the btn-brand class to "Export results to CSV" and "Download CSV" to reuse the styling from LMS.
3. Removes the "Submit" button from the results page, as it's redundant there. All fields and the button itself are disabled anyway.
4. Moves the versioning to poll/__init__.py to match other XBlocks.
5. Adds load_requirements to setup.py to use the correct requirements.
6. Removes six usage from the XBlocks and tests.
7. Updates xgettext-template to eliminate potential vulnerabilities.
8. Adds support for xblock.utils (Remove xblock-utils dependency #114). Does not remove the support for xblock-utils, though.
9. Adds the MANIFEST to correctly include files in the Python package. You can see that these flies were missing by checking that the README is not displayed in PyPi.

Screenshots

Before

After

Instructions

1. Set up the XBlock in Studio (as described in the README) and check its basic functionality in the LMS/Preview.

Private-ref: BB-8763

[xitij2000]

👍 Looking good! Thanks for the cleanup and modernisation.

• I tested this: tested on tutor devstack
• I read through the code