GitLab Pages force HTTPS
### Description
GitLab Pages provides both HTTP and HTTPS connections (if a certificate is available), but HTTPS is not enforced. So, if you request resources via HTTP, it will stay unencrypted.
### Proposal
Add an option that enforce HTTPS connections: if a connections is HTTP, and the current domain (shared or custom) has a certificate, a `301` redirect message is sent back to the client.
```
HTTP/1.1 301 Moved Permanently
Location: https://<username>.gitlab.io
```
This setting is per project.
### Design proposal
* Add checkbox to enable force redirect to HTTPs to Settings > Pages.
* Setting is saved automatically.
* This will have HTTPS only by default for all new pages, off for existing ones

issue