Set up Bootstrap SASS compilation and customize theme

What does this MR do?

I have set up a way to customize the Bootstrap distribution we use. This paves the way for theme customization, since we can now modify its variables. In this PR, I make a basic customization of the theme, but more advanced things can follow later!

I had to go through a lot of hoops to make this work. First, there are tons of SASS Gradle plugins, but only one of them works as advertised (with some effort). This plugin however does not allow to import SASS files from the classpath, so I had to find where the webjar of bootstrap is located and copy it into the directory. The whole of CSS needs to be copied into a new directory which can be detected by the server. I tried very hard to make this a directory that would be hidden inside some build directory, but in the end, the only thing that worked was making a new public directory for it.

Screenshots

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.