[web] Rename MessageActionButtons to MessageTooltip

Summary: Relevant Linear issue here. @ashoat and I discussed this today and determined that since the timestamp was moved into MessageActionButtons (see D3571) it doesn't make sense that the component called MessageActionButtons contains the action buttons and the timestamp. Instead, renamed it to something that better encapsulates the component, which is a tooltip that opens when a user hovers over a message or RobotextMessage.

Test Plan: Replaced all occurrences of MessageActionButtons, messageAction that had to do with MessageActionButtons. Did not replace the instances of messageActionButtons in the CSS for MessageActionButtons because those were actually pertaining to the message ation buttons. Everything else (like in ComposedMessage or RobotextMessage) were just referring to the tooltip as a whole.

Reviewers: atul, palys-swm

Reviewed By: palys-swm

Subscribers: Adrian, ashoat

Differential Revision: https://phab.comm.dev/D4426