This reverts merge request !28545

This is a CSS effort only.

Fixes the padding of the buttons to be `8px 12px`
(including border) so that the button height is
32px.

Also adjusts the border width for all buttons when
the state is `hover`, `focus`, or `active to be 2px
thick instead of 1px thick. This is accomplished
through an inset box-shadow as not to increase
the size of the button.

Fixes some colors for primary and tertiary buttons.

Added stylelint to static-analysis
Updated yarn dependencies
CSS Fixes and rule adoptions of stylelint
Added stylelint-scss
Deduplicated yarn.lock to clear dependencies

First round of advanced stylelint rules

Mainly Vendor prefix updates related to flex

Updates to more webkit specific vendor prefixes

Finished all vendor specific fixes

Moved now 4 rules to warning

Fixed the new scss lint problems

More stylelint adaptions after rebase

Rendering ten thousands of tree items consumes a lot of server
time and can cause timeouts in extreme cases. Realistically,
displaying more than 1,000 files is probably not useful so
truncate and show the user a notice instead. 'Find files' can be
used to locate specific files beyond the 1,000 limit.

FE for https://gitlab.com/gitlab-org/gitlab-ce/issues/34102

Fix
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12198#note_37146360

hide placeholder text when files have been added
vertically center dropzone area content
close modal after file upload succeeds

Closes #34427

- Move the find_file_link and download partial into the project tree_header partial
- Added Changelog

center spinner
show placeholder text on new MR page
remove redundant `display: block` from li - they default to `list-style` already