back to two verification-check functions (#4966)

this pr keeps and refines documentation added in #4951, however, reverts
the api introduced by #4951
which turns out to be not useful for UI in practise:

UI anyway check for chat/no-chat beforehand,
so a simple condition in profiles as
`green_checkmark = chat_exist ? chat_is_protected() :
contact_is_verified()`
is more useful in practise and is waht UI need and did already in the
past. (https://github.com/deltachat/deltachat-android/pull/2836 shows a
detailed discussion)

(as a side effect, beside saving code,
this PR saves up to three database calls
(get contact from chat in UI to pass it to profile_is_verified(), get
chat from contact in core, load is_protected in core) - instead, core
can use already is_protected from already loaded chat object)

/me did check rust-tests, fingers crossed for python tests
/me should re-setup python tests on local machine at some point :)

deltachat-jsonrpc/src/api/types/contact.rs

@@ -24,8 +24,9 @@ pub struct ContactObject {
     ///
     /// If this is true
     /// UI should display green checkmark after the contact name
-    /// in contact list items and
-    /// in chat member list items.
+    /// in contact list items,
+    /// in chat member list items
+    /// and in profiles if no chat with the contact exist.
     is_verified: bool,
 
     /// True if the contact profile title should have a green checkmark.