Add gorilla/csrf middleware to protect all HTML-serving routes against
cross-site request forgery attacks. The webhook endpoint is excluded
since it uses secret-based authentication.
Changes:
- Add gorilla/csrf v1.7.3 dependency
- Add CSRF() middleware method using session secret as key
- Apply CSRF middleware to all HTML route groups in routes.go
- Pass CSRF token to all templates via addGlobals helper
- Add {{ .CSRFField }} / {{ $.CSRFField }} hidden inputs to all forms
Closes#11
