Build css from scss sources in client ui build

[pbankonier]

Description

The following things are done in this PR:

• Add the compiling of the scss sources to css to the maven build for the client ui. I used this (https://github.com/eirslett/frontend-maven-plugin) maven plugin for that.

• This was required to remove the css files from the project. This is necessary, because it happened multiple times that the css files were modified directly instead of editing and then compiling the scss files, which results in a mismatch between the scss src and the compiled css.
  e.g. https://github.com/apache/cloudstack/pull/3240/files

• I also moved the ui folder with the frontend resources inside the client ui module folder because they got only used there and it is a better location for them.

• This (compiling scss to css during the build) is the currently preferred way of handling these frontend resources.

• This changes are not affecting any functionality nor are a breaking change.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

How Has This Been Tested?

Deployed this to our test environment and tested functionality.