Added loading spinner in Popup.html

Fixes #52 (closed)

Short description of what this resolves:

whenever the url matches and the user try to register the Scholar then extension takes some time to register the profile.

Changes proposed in this pull request:

• added some css animation in style.css file
• Added logic to spinner for doing on or off in popup.js file
• Added html tag for the spinner

Checklist

• My branch is up-to-date with the Upstream develop branch.
• The acceptance, integration, unit tests and linter pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)