Added support for refreshing the device list, marking current device with a checkmark instead of a lock

Mentions are now an '@' icon in the upper right.

UI Popup is now a smaller dialog.

Still lots of work to be done here.

This reverts commit 4b807229.

fixes #235

fixes #204
fixes #255

* Validate both inferred and explicitly entered server addresses by attempting to call the /versions endpoint
* If the domain from the mxid fails validation, try prefixing it with 'matrix'
* Only show server address field if address validation ultimately fails

Logout from the current session and invalidate the current token