Implement flickity

[grazirs]

What github issue is this PR for, if any?
Resolves #54

What changed, and why?

• Now, all the sliders on the website use the Flickity library instead of OwlCarousel2 and Slick libraries.
• Also, I fixed the text animations in the home page slider.

Sliders:

• The hero slider on the home page
• The supporters slider on the home page
• The testimonial slider (not currently rendered on the site, but you can find an example here)
• The project slider (not currently rendered on the site -->example:)
• Clean up main.js
  Hero slider and supporter slider, you can see here, the last two are not rendered on the site yet.

sliders-1.mov

[meg-gutshall]

This looks great! Your code is super clean as well!

One change request and then I'll merge it in. Can you slow down the speed of the sliders? The main one we had at 20000ms and the supporters at 5000ms.

[meg-gutshall]