Replace View Instance with instance dropdown

[davkal]

• makes it clear what you're viewing
• allows to quickly change the instance
• instance management icon was removed, can be reached via dropdown

Fixes #590

[foot]

• Whaddaya think about getting the cog for the instance away from the profile and over w/ the instance?
• pointer cursor on the dropdown items would be cool. (is this mobilefirst popping up? why doesn't matui do this, is there a global flag/setting we should be using...) Ayyyyy! https://github.com/callemall/material-ui/blob/v0.15.3/CHANGELOG.md#0153
• With manage instances losing its prominence, Create new instance is a bit hard to find now, maybe we could put that in the drop down too.
• Active item in the dropdown diff color/inactive (maybe).

A little related but feels more prominent now

• Flash of loading when you click the accounts/setting icon, would be nice to hold onto the state across page changes (and perhaps still request and update it if its changed).

[foot]

Code LGTM

[davkal]

Implemented feedback. Loading instance and identity on page change needs to go in separate PR.

[foot]

Nice one! LGTM