IDE: Load full Files structure through API

What does this MR do?

This loads the structure of the whole branch through the files api (which is already in use for the existing File Finder).

• Reduces the loading time of the file tree
• Loads the whole file tree at once
• Enables deep loading (Loading of files that are in subdirectories)
• We will reimplement Lock Status + Submodules through additional calls

Are there points in the code the reviewer needs to double check?

• Loading Logic + Creation of the Tree at once recursively
• File Tree is now also rendered as one recursive control and not a flat structure
• Flexboxed the file component

Why was this MR needed?

Pre-Requisite for the Fuzzy File Finder in the Web IDE + MR diff display

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Pre Requisite for gitlab-org/gitlab-ee#4571