Makes changes to dropdown-typeahead2 so that a css
class for the button can be passed in. Means it can
be used instead of dropdown-typeahead.

Switches to using dropdown-typeahead2 for series_overrides
directive and for the influxdb, mysql and postgres datasources
as it already contains a fix for this issue.

This commit also fixes the index property which
was set using an incorrectly spelled length property in the
series_overrides directive.

Closes #15621