Improvements for the vpn client UI

[Senyoret1]

Did you run make format && make check?
The go code was not changed. npm run lint and npm run build were used.

Changes:

• The UI now shows the connection time.
• Some improvements were added to the coded which decides when to update the IP info.
• Now the vpn started UI shows the errors returned by the /api/visors/{pk}/apps/vpn-client/connections API endpoint.
• Various problems when no server has been selected were solved.
• A generic error msg was added for when the API does not return the details of an error.
• An error indication is now shown when there is a problem updating the status of the app.

How to test this PR:

• Connection time: select a server and start the vpn. The connection time will appear at the top of the current connection status. It is updated when the VPN is connected.
• IP info: the ip should be updated when entering and leaving the connected state.
• Errors returned while connected: not sure about how to tests this in the wild.
• Problems when there is no server selected: remove the server in the vpn-client section of the visor config. The UI should say that there is no server selected and you should be able to continue using the UI notmally (like selecting a server and connecting to it).
• Error indication: use the dev tools of the browser, open the network tab and block the network connections. A warning will appear at the top right corner of the UI.

[Senyoret1]

The last commit fixes a problem which made the server name to go outside the box boundaries if it is too long.

[Senyoret1]

The last commit includes the following changes:

• Now, when a server does not have a name, Unnamed is shown as the server name, instead of the server pk.