Consolidate .gitattributes at the top level

[cbandy]

This moves our single .gitattributes file to the top-level file so it is recognized in GitLab, too. At the top level, it can also define macros that combine multiple attributes.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• Other