Support additional ad-hoc filters

[RoryCrispin]

Ad-hoc filter variables can be added to a dashboard and used to filter the ClickHouse data source results.
The ad-hoc filter UI offers

=
!=
>
<
=~
!~

But the CH datasource does not support the regex expressions ~= and !~
Either, the UI should hide these fields, or support for them should be added.

I'm not sure if the plugin has support for hiding the options, however.

[mshustov]

= and !

It's a bug. Created a bug fix #414