feat: add GitHub edit and issue buttons to curriculum pages (#552)

[SHENIPRASHANTH]

Fixes #552

Added two helpful links at the bottom of each curriculum page:
1)"Edit this page on GitHub"
2)"Open an issue for this page"

This makes it easier for contributors to propose changes or report problems.

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[illicitonion]

Hi! Thanks for putting this together!

Can you talk me through how you're testing this change locally? I tried checking out your branch an running the Intro To Programming site (by running: cd org-cyf-itp && npm run start:dev) and it gave me an error:

ERROR render of "taxonomy" failed: "/Users/dwh/src/github.com/CodeYourFuture/curriculum/common-theme/layouts/_default/baseof.html:23:82": execute of template failed: template: _default/taxonomy.html:23:82: executing "_default/taxonomy.html" at <.File.Path>: error calling Path: runtime error: invalid memory address or nil pointer dereference

I think this generally looks good, but I'm not sure it quite always works at the moment :)