Draft: Resolve "Migrate upload license page to Vue"

What does this MR do and why?

Migrate upload license form to Vue

Resolve #352501

Screenshots or screen recordings

Old behavior:
Screencast_from_05-21-2022_01_58_42_PM

New behavior:
Screencast_from_06-05-2022_11_50_09_PM

How to set up and validate locally

1. Visit /admin/application_settings/general
2. Expand "Add License"
3. Upload license

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Changes

• Gitlab API is used (instead of private ruby controllers)
• File is not sent to server, FileReader is used to read content
• Page is not refreshed, success and error messages are shown using flash

TODO

• Get answer from UX to the following questions:

• Is it OK to use flash success message? Current realization redirects to subscription plan page.

• What the message should be? I guess I need to add a subscription plan link to the message.

• Remove deprecated createFlash from tests

• Change e2e tests (now they expect page reloading (when error occurs) and redirect (when license is valid)

Related to #352501, #359743 (closed)