Add http and https proxy feature

Added new proxy feature based on http and https proxy agents.
Proxy feature works like notifications, there is many proxy
could be related one proxy entry.

Supported features
- Proxies can activate and disable in bulk
- Proxies auto enabled by default for new monitors
- Proxies could be applied in bulk to current monitors
- Both authenticated and anonymous proxies supported
- Export and import support for proxies

src/languages/en.js

@@ -369,4 +369,12 @@ export default {
     alertaApiKey: 'API Key',
     alertaAlertState: 'Alert State',
     alertaRecoverState: 'Recover State',
+    Proxies: "Proxies",
+    default: "Default",
+    enabled: "Enabled",
+    setAsDefault: "Set As Default",
+    deleteProxyMsg: "Are you sure want to delete this proxy for all monitors?",
+    proxyDescription: "Proxies must be assigned to a monitor to function.",
+    enableProxyDescription: "This proxy will not effect on monitor requests until it is activated. You can control temporarily disable the proxy from all monitors by activation status.",
+    setAsDefaultProxyDescription: "This proxy will be enabled by default for new monitors. You can still disable the proxy separately for each monitor.",
 };