Add feature to view users messages/offers and send private message from user profile info

[HenrikJannsen]

The user profile info sidebar should be converted to a popup and gets added the feature to see all messages and offers of that user.
We should add the option to send a private message to a user.

Currently when all views at the offerbook are open we have 5 columns including the leftnav, which is too much.
We should move the user info and channel info to a popup.
Channel info could also be split into a menu overlay for notifications and the channel members list as popup.

Element use that overlay for channel notifications:

I think a similar approach might be good.

[HenrikJannsen]

Overlap with #1941. Will close #1941 as this issue is has more details and more features.