Sunsetting crowdAI banner & remove signup page

[scarroll32]

crowdAI will soon be archived, and users will be notified to migrate to AIcrowd instead.

• add a banner at the top of every page with the text: "crowdAI is shutting down - please read our blog post for more information" with a link to https://www.crowdai.org/blogs/7
• remove ability for users to create new accounts
• existing users will still be able to log in and access all of the site
• grading infrastructure left in place for now (inc. GitLab).

cc: @marcelsalathe @spMohanty

[scarroll32]

https://github.com/crowdAI/crowdai/commit/d8dcc91e16ffbe6a37ffc5cedecc20b9b207be29

[scarroll32]

Deployed