Bug/01 open wallet same browser

This PR fix an old issue regarding to open a same wallet in the same browser.

* Added a warning page before open a wallet with two options: signout or proceed anyway.
* This feature use a flag in the localStorage: walletId::isOpen -> true.
* Karma test: unlock the wallet to continue even if it is already open.
* Mocha test: open the same wallet twice. 

Known issues

1. Open a wallet. 
2. Open same wallet in another browser tab. Click on proceed anyway, and close it.
3. Now you can open the same wallet without any warning.