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/SidePane/AccountDelegate.qml

@@ -54,6 +54,7 @@ Column {
                     placeholderText: qsTr("Set status message")
                     font.pixelSize: theme.fontSize.small
                     background: null
+                    bordered: false
 
                     padding: 0
                     leftPadding: accountLabel.leftPadding