Add build-css step on build script

[patitonar]

Right now if you want to add/change a css style, you have to update the corresponding .scss file, then run npm run build-css to generate the compiled css file src/assets/stylesheets/application.css and then it need to be included in git with the rest of the changes since the react app uses that file to apply the styles. This can be confusing specially for new contributors (see this comment)

This PR removes the application.css file from git and updates start and build scripts to run build-css as a first step so the file application.css will be generated before the react app builds.

So after these changes, changing a css style will only require updating the .scss file

[netlify[bot]]

Deploy preview for kind-kilby-95344f processing.

Building with commit 73dcdf6fe02aff6dbc0fe3a131f0a1135188171f

https://app.netlify.com/sites/kind-kilby-95344f/deploys/5c6ec14edb27110008db85cc

[coveralls]

Pull Request Test Coverage Report for Build 334

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 3.232%

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Totals
Change from base Build 328:	0.0%
Covered Lines:	46
Relevant Lines:	1090

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💛 - Coveralls