Proxy: Make Proxy-Screen accessible from Chat-list

[zeitschlag]

• [x] Add a shield to Chat-list's NavigationBar if proxy is enabled (and a shield.slash if it's broken, not enabled, ...)
• [x] Show Proxy-Screen when tapping on that shield
• [x] if iOS 13+ use SF Symbol else use Text Proxy

[r10s]

Add a shield to Chat-list's NavigationBar if proxy is enabled (and a shield.slash if it's broken, not enabled, ...)

ftr, this is what android is doing - as well as https://github.com/deltachat/deltachat-ios/pull/2391 at a first glance:

• if proxy_url is not empty and we're not in relay-mode, show a button, independent of proxy_enabled
• show button as disabled (so, shield.slash) in case proxy_enabled is false

connectivity does not play a role for the button - that seems to be fine, as the "connected" state is shown just beside.