Add validation for saveWebClientState, postMovie, and postScreenshot

[dgarciabriseno]

This change adds a json schema validator to strictly validate the contents of the web client state being saved, and validate the given arguments for postMovie and postScreenshot.

This is implemented via a new "schema" validator on the existing InputValidator.
JSON schemas should be placed in the management/schema and then copied to docroot/schema via management/schema/deploy.sh.

• Schema files in the management/schema may contain template parameters populated by the database.
• Schema files in docroot/schema are final schema files that may be accessed globally by any json schema validator (not only helioviewer).

To validate a json field, the input to the validator looks like this:

$expected = array(
  'schema' => array('json' => 'https://api.helioviewer.org/schema/some_schema.json')
)

Where the first schema key tells the validator to use the schema validator, and the second array is in the form input_parameter_key => schema_file.