Staff directory ADA issues

[bbusenius]

Fixes #671

Changes in this request

• Removed redundant headings in staff listing in directory and changed the heading structure here.
• SCSS changes related to the above.
• Removed alt text on placeholder images in the staff directory.
• Removed alt text on other placeholder images throughout the site.
• Made "visit our locations" text in sidebar more accessible by moving it to an aria label.
• Linted all related templates and fixed invalid HTML errors.

To view changes without linting whitespace differences, run this command when on the working branch:

git diff -w master

Make sure the updated CSS displays:

1. Run ./manage.py compress
2. Restart the Django dev server.
3. Empty cache in the admin and from the shell both.