WIP: Design Tab: Drag and drop upload of Designs

What does this MR do?

Feature proposal: #39452 (closed)

TODO:

• Make dropzone clickable
• dropzone specs

Demo video

Currently demos dragging over the "empty state" card ONLY (not over the design cards themselves).

NOTE: this demo is slightly out of date, and does not include latest improvements made by @jareko

Kapture_2020-02-24_at_17.05.46

Screenshots

Error	Screenshot
Unsupported file type	Reveal
Not dragging files	Reveal

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #39452 (closed)