Move source files into a src/ directory

Currently, GitLab UI's code is scattered across multiple directories that are placed in the project's root, next to Storybook-specifics, tests, compiled output etc.

Let's move everything that's closely tied to GitLab UI's compiled bundle into a src/ directory, this will make the root directory easier to grasp, and we would be able to simplify our usage of require.context to match all stories in a single call (see !810 (comment 225957170)).

/cc @dpisek