Ask for server URL before showing sign in box

Contact the server's .well-known API before anything to get
available login flows instead of blindly assuming it will be
m.login.password, and to get the server's real URL instead of
requiring users to remember that e.g. it's "chat.privacytools.io"
and not just "privacytools.io" despite user IDs making it look like so.

The server field will also now remember the last accepted URL.

src/backend/backend.py

@@ -6,14 +6,14 @@ import os
 import sys
 import traceback
 from pathlib import Path
-from typing import Any, DefaultDict, Dict, List, Optional
+from typing import Any, DefaultDict, Dict, List, Optional, Tuple
 
 from appdirs import AppDirs
 
 import nio
 
 from . import __app_name__
-from .errors import MatrixError
+from .errors import MatrixError, MatrixForbidden, MatrixNotFound
 from .matrix_client import MatrixClient
 from .media_cache import MediaCache
 from .models import SyncId
@@ -129,6 +129,30 @@ class Backend:
 
     # Clients management
 
+    async def server_info(self, homeserver: str) -> Tuple[str, List[str]]:
+        """Return server's real URL and supported login flows.
+
+        Retrieving the real URL uses the `.well-known` API.
+        Possible login methods include `m.login.password` or `m.login.sso`.
+        """
+
+        client = MatrixClient(self, homeserver=homeserver)
+
+        try:
+            homeserver = (await client.discovery_info()).homeserver_url
+        except (MatrixNotFound, MatrixForbidden):
+            # This is either already the real URL, or an invalid URL.
+            pass
+        else:
+            await client.close()
+            client = MatrixClient(self, homeserver=homeserver)
+
+        try:
+            return (homeserver, (await client.login_info()).flows)
+        finally:
+            await client.close()
+
+
     async def login_client(self,
         user:       str,
         password:   str,

src/backend/matrix_client.py

@@ -152,11 +152,13 @@ class MatrixClient(nio.AsyncClient):
     }
 
 
-    def __init__(self,
-                 backend,
-                 user:       str,
-                 homeserver: str           = "https://matrix.org",
-                 device_id:  Optional[str] = None) -> None:
+    def __init__(
+        self,
+         backend,
+         user:       str           = "",
+         homeserver: str           = "https://matrix.org",
+         device_id:  Optional[str] = None,
+    ) -> None:
 
         if not urlparse(homeserver).scheme:
             raise ValueError(