DTIP-12 Replace background logo and login-screen

[khiemampsight]

Generic requirements in order to contribute to MISP:

• One Pull Request per fix/feature/change/...
• Keep the amount of commits per PR as small as possible: if for any reason, you need to fix your commit after the pull request, please squash the changes in one single commit (or tell us why not)
• Always make sure it is mergeable in the default branch (as of today 2020-05-05: branch 2.4)
• Please make sure Travis CI works on this request, or update the test cases if needed
• Any major changes adding a functionality should be disabled by default in the config

What does it do?

If it fixes an existing issue, please use github syntax: #<IssueID>

Questions

• Does it require a DB change?
• Are you using it in production?
• Does it require a change in the API (PyMISP for example)?