Change HTextField borders, SignIn disable buttons

By default now, fields have a transparent border if not focused and
accent-color border if focused.
Set `bordered: false` for no border at all.

The register and forgot buttons on the SignIn screen are now disabled,
since their functions aren't implemented yet.

src/qml/Chat/RoomSidePane/MembersView.qml

@@ -43,6 +43,7 @@ HColumnLayout {
         id: filterField
         placeholderText: qsTr("Filter members")
         backgroundColor: theme.sidePane.filterRooms.background
+        bordered: false
 
         Layout.fillWidth: true
         Layout.preferredHeight: theme.baseElementsHeight