Conversation actions layout redesign

As part of making the interface adaptive we'll need to revamp the conversation actions somewhat. While we're at it, it would be good to clean up the UI for this more generally.

Issues with the current conversation actions layout:

• The "mark conversation" menu is basically a secondary menu in disguise, and not very discoverable. It's also grouped in with labels and folders, which I feel is not a great fit.
• The "mark conversation menu" contains double entries for everything (e.g. both star and unstar)
• The undo button is awkward and should probably just be replaced by an in-app notification.
• The primary actions on the thread (mark as read, archive, delete, mark as spam) are spread out across different places, and all look different (some are menu entries, some are icon buttons, some icon+text buttons)

I mocked up some different options for addressing these issues, making it cleaner and more adaptive-friendly in the process:

Some of the ideas I tried out here:

• Move "Star" next to the title to have fewer things in the toolbar(s)
• Move "Reply" et al. inline after the last message, and use text buttons instead of icons
• Add a real secondary menu for less important actions
• Always have an action bar to keep the same layout between desktop and mobile