General Improvements: Create and modify data source at runtime

[leandrocp]

Beacon live data source is currently defined as a module in the host application but that impose a couple of problems:

1. Require Elixir knowledge to change values, even scalar basic values like a person name or a number;
2. Require redeployment, which goes against Beacon's main proposal;
3. Templates get out of sync causing crashes, suppose you change an assign name in a template. That change requires changing the data source but the template will break until the data source is deployed, and doing the operation operation (changing the data source first) will also crash the template until the updated page is published.

For those reasons, we need to to:

• Add a new tab in the Pages page to allow users to create/edit the data source for a given page;
• Version the data source along with the template (the whole page) so changes on either one goes together;

The layout for that page may contain:

• The assign name
• Data type (text, number, elixir code)
• Content rendered as https://github.com/BeaconCMS/live_monaco_editor

The data type allows us to display the correct "content" field.