Warn when using an unencrypted (non-HTTPS) server connection (Add connection security status icon)

First, Nextcloud is awesome! Keep up the good work!

### Actual behaviour
When using the Nextcloud app to connect to a Nextcloud server over an unencrypted connection, the user is not warned that the connection is **not** secure. This does not happen at first nor subsequent logins.

### Expected behaviour
When a user attempts to connect to a Nextcloud server over an unencrypted connection, the user should be warned that their connection is not protected/private, and that their data is at risk.
Currently the app **does** display this kind of warning when connecting to a server that has a **self-signed** certificate. So I think it makes sense to also warn when encryption is not in use at all.

One could argue that a user should already know if a server is using encryption based on the connection information they enter, or that Nextcould should never be available over an unencrypted channel, and I would have to agree. But I still think they deserve the warning.

#### UI Suggestion:
Add an icon or some other indication that the connection to the server is secure.
or
Add an icon or some other indication only when the connection is **not** secure.

### Steps to reproduce
1. Set up Nextcloud, but do not configure encryption (https) on the web server.
2. As admin, set up a new user.
3. Use the Nextcloud Android app to connect to the server and log in for the first time as the new user.
4. Observe that no warning is presented to the user regarding the lack of connection security.
5. After the initial log-in, restart the app to reconnect.
6. Observe that no warning is presented.

### Environment data
Android version: **6.0**

Device model: (**Generic**)

Stock or customized system: **Stock**

Nextcloud app version: **3.3.0**

Nextcloud server version: **14**