Few changes -
* webview: Show connection failure per server.
* network: Try to reconnect diff servers.
* Fixes concern that some proxy networks may allow only specific servers
to be reachable.
* domains: Show network error on server invalidation.
* webview: Handle network errors in preload script.
Fixes: #591, #312.
* lint: Implement CSS linting with stylelint.
* lint: Format CSS files according to lintconfig.
Adds CSS linting following the rules of zulip webapp.
Some rules have been ommitted because they required
significant changes in code. Also, creates a separate
.stylelintrc for storing css config.
Fixes CSS part of #676.
