adds validation for fields.

* renames RoomTooltip to be a generic Tooltip (which it is)
 * hooks it into Field to show validation results
 * adds onValidate to Field to let Field instances call an arbitrary validation function

Rebased from @ara4n's https://github.com/matrix-org/matrix-react-sdk/pull/2550
by @jryans. Subsequent commits revise and adapt this work.

res/themes/dark/css/_dark.scss

@@ -53,6 +53,7 @@ $input-lighter-bg-color: #f2f5f8;
 $input-lighter-fg-color: $input-darker-fg-color;
 $input-focused-border-color: #238cf5;
 $input-valid-border-color: $accent-color;
+$input-invalid-border-color: $warning-color;
 
 $field-focused-label-bg-color: $bg-color;