Bug 1022644 - [Messages] Can't open the recipient panel if there are only 2 lines of recipients

- Remove unnecessary property dims and replace it with minheight element style
Yeah, agree. ```min-height``` should be defined in the ```#messages-to-field``` rule, moreover I'm wondering why ```section[role="region"].new #messages-to-field``` has any styles other than ```display:flex``` at all? As far as I understood this rule is intended just to display "To" field when in composer(new) panel.