Add voice/SMS contacts with Twilio

This patch adds user-configurable SMS and text-to-speech voice alerts using [Twilio](https://www.twilio.com), a programmable voice and SMS service. I use this service for iPTF alerts.

This is a cheaper and more powerful alternative to the [Email2Phone](http://www.email2phone.net) service that is currently in use. The pricing for Twilio is $1/month, plus $0.015/minute voice and $0.0075/text message. This is about 1/20 to 10/10 the price of Email2Phone depending on volume.

Note that we do need to figure out who would pay for this service before merging it. That may depend on how much it costs, which in turn depends on how many people use it. If we have about 10 alerts a month, all going to about 20 follow-up advocates, then the service would cost $3.25 / month or $39 / year of continuous operation. If a significant fraction of the collaboration used it (about 500 people), then it would be $113.50 / month or $1362 / year. Another way to think of it is that it would cost on the order of a dollar per year per person who uses the feature.

## TO DO

Note that we should probably restrict this feature to LVC users only (i.e., not EM partners). @tanner.prestegard should comment on how to test for LVC membership before issuing alerts.

## How to set up phone alerts

Please do NOT overuse this test script. It WILL cost me money.

Once you are logged in to GraceDB, navigate to the `OPTIONS` tab:

![screenshot1](/uploads/ac8c816335589ec2b0895de752fa889f/screenshot1.png)

Then click `Create New Contact`. In the form, enter a description and a phone number, then click `Submit`:

![screenshot2](/uploads/3d48d7b5bf22e1b6760c3d95c1be0965/screenshot2.png)

Click `Create Notification`. Select `ADVREQ` from the `Labels` list and your new contact from the `Contacts` list, then click `Submit`.

![screenshot3](/uploads/01f832734f55e79cd0a102b546897168/screenshot3.png)

You have now created a voice/SMS notification that will be triggered whenever the `ADVREQ` label is applied to an event.

![screenshot4](/uploads/698834131791accb20a2cbbf623e8956/screenshot4.png)

See merge request !1

Twilio updates

Added a check for LVC membership before making phone calls. Moved code which makes the phone calls into its own function. Moved Twilio account SID, auth token, and URL hashes into a secret settings file (encrypted and deployed via Puppet).  Also added a help text note on the Create Contact page.

See merge request !1

Implemented new VOEvent parameters and allowed preliminary VOEvents to include skymaps (although they aren't required to).