Gracefully handle websocket disconnection in web terminal

Description

Currently when Workhorse closes the websocket connection the web terminal becomes just unresponsive - the text that I type in does not appear in the screen. In the Chrome developers console I get:

WebSocket is already in CLOSING or CLOSED state.

We should at least provide some feedback to the user or, ideally, attempt to reconnect.

Proposal

1. Display a message that connection was terminated
2. Try to reconnect

UX

Links / references

• https://docs.gitlab.com/ce/administration/integration/terminal.html
• https://gitlab.com/gitlab-org/gitlab-ce/issues/22864
• gitlab-workhorse#86 (closed)

/cc @nick.thomas @fatihacet