Move invite option to header of members section

This patch moves the invite option to header of members section. We also check if user can send invitation event and we won't show the invite button if it's not allowed. (ps. seems the NeoChat matrix room #neochat:kde.org doesn't allow sending invitation?)

This patch also added an toggle button to show the member search bar since it won't needed by the user in most of the cases.

See the following image: