Initial tox and gitbub actions: avoid getting stale css

[OmarIthawi]

Changes

• Add tox to compile SCSS files easily via tox -e compile
• Add GitHub actions to block pull requests from unless the CSS file is compiled

Partially fixes #91

What does the error look like?

More details?

This what the GitHub actions shows when it fails:

Error: CSS is out of sync
=== Start: Git diff ===
diff --git a/cms/static/css/amc-specific.css b/cms/static/css/amc-specific.css
index 61c5a1e..2d4bb1d 100644
--- a/cms/static/css/amc-specific.css
+++ b/cms/static/css/amc-specific.css
@@ -99,3 +99,6 @@ body.view-settings .group-settings.basic .list-actions {
     max-width: 48rem;
     width: 80%;
     margin-bottom: 5rem; }
+
+.xblock .mentoring .copyright {
+  display: none; }
=== End: Git diff ===
Error: The CSS file is out of sync with the SCSS file.
The git diff above shows the difference.
Please use "$ tox -e compile" locally to compile the SCSS file.
ERROR: InvocationError for command /bin/bash scripts/verify.sh (exited with code 1)
____________ summary ____________
ERROR:   verify: commands failed