Warn when Geo replication is proceeding over HTTP, rather than HTTPS.

The following discussion from !3116 (merged) should be addressed:

• @brodock started a discussion: (+2 comments)

  I understand that this is an identifier only, but shall we force it to always be HTTPS otherwise go back to SSH (to make sure communication is always encrypted)?

Currently, file sync (attachments, LFS objects, etc) can proceed over an unencrypted HTTP connection. From %10.2, repositories can be synced using the same mechanism.

I think it's reasonable to specify that Geo primaries must use HTTPS. WDYT @jramsay? 10.2 or 10.3?

We can add a warning banner when Geo is configured insecurely, recommending that admins upgrade to HTTPS from HTTP.

Stretch: can we detect whether postgresql replication is secured and display that per Geo secondary?