replaces the spinner GIF with CSS

[bashir2]

Description of what I changed

Fixes #420

Measuring approximate CPU usage (the controller UI was on a remote machine, connected through Chrome Remote Desktop; these are based on CPU usage on the local machine):

• Before the change, i.e., with the animated gif, CPU usage increased by 15-20% while showing index.html.
• After the change, i.e., with the CSS version, CPU usage increased by ~3%. while showing index.html.

E2E test

TESTED:

Ran the controller locally, also checked the static index.html with spinners turned on and the CPU usage was much lower than the old animated GIF.

Checklist: I completed these to help reviewers :)

• [x] I have read and will follow the review process.

• [x] I am familiar with Google Style Guides for the language I have coded in.

  No? Please take some time and review Java and Python style guides.

• [x] My IDE is configured to follow the Google code styles.

  No? Unsure? -> configure your IDE.

• [ ] I have added tests to cover my changes. (If you refactored existing code that was well tested you do not have to add tests)

• [x] I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.

• [x] All new and existing tests passed.

• [x] My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master

[chandrashekar-s]

@bashir2 Can you also please add the difference in the CPU utilization in the description?

[bashir2]

@bashir2 Can you also please add the difference in the CPU utilization in the description?

Updated PR description with CPU usage changes.