auth: Rewrite data model for tracking enabled auth backends.

So far, we've used the BitField .authentication_methods on Realm
for tracking which backends are enabled for an organization. This
however made it a pain to add new backends (requiring altering the
column and a migration - particularly troublesome if someone wanted to
create their own custom auth backend for their server).

Instead this will be tracked through the existence of the appropriate
rows in the RealmAuthenticationMethods table.

zerver/lib/import_realm.py

@@ -51,6 +51,7 @@ from zerver.models import (
     Reaction,
     Realm,
     RealmAuditLog,
+    RealmAuthenticationMethod,
     RealmDomain,
     RealmEmoji,
     RealmFilter,
@@ -77,6 +78,7 @@ from zerver.models import (
 )
 
 realm_tables = [
+    ("zerver_realmauthenticationmethod", RealmAuthenticationMethod, "realmauthenticationmethod"),
     ("zerver_defaultstream", DefaultStream, "defaultstream"),
     ("zerver_realmemoji", RealmEmoji, "realmemoji"),
     ("zerver_realmdomain", RealmDomain, "realmdomain"),
@@ -105,6 +107,7 @@ ID_MAP: Dict[str, Dict[int, int]] = {
     "subscription": {},
     "defaultstream": {},
     "reaction": {},
+    "realmauthenticationmethod": {},
     "realmemoji": {},
     "realmdomain": {},
     "realmfilter": {},
@@ -598,19 +601,6 @@ def fix_bitfield_keys(data: TableData, table: TableName, field_name: Field) -> N
         del item[field_name + "_mask"]
 
 
-def fix_realm_authentication_bitfield(data: TableData, table: TableName, field_name: Field) -> None:
-    """Used to fixup the authentication_methods bitfield to be an integer."""
-    for item in data[table]:
-        # The ordering of bits here is important for the imported value
-        # to end up as expected.
-        charlist = ["1" if field[1] else "0" for field in item[field_name]]
-        charlist.reverse()
-
-        values_as_bitstring = "".join(charlist)
-        values_as_int = int(values_as_bitstring, 2)
-        item[field_name] = values_as_int
-
-
 def remove_denormalized_recipient_column_from_data(data: TableData) -> None:
     """
     The recipient column shouldn't be imported, we'll set the correct values
@@ -955,7 +945,6 @@ def do_import_realm(import_dir: Path, subdomain: str, processes: int = 1) -> Rea
     # Fix realm subdomain information
     data["zerver_realm"][0]["string_id"] = subdomain
     data["zerver_realm"][0]["name"] = subdomain
-    fix_realm_authentication_bitfield(data, "zerver_realm", "authentication_methods")
     update_model_ids(Realm, data, "realm")
 
     # Create the realm, but mark it deactivated for now, while we