add internal hook points support with scripting capabilities

[syjer]

for example:

• when a ticket is bought, execute a script (javascript) that will do a http post to remote interface.
• expose a generic web hook end point for handling incoming requests with a user defined script -> will be done in another task

[syjer]

TODO:

• wire up the events + script execution for async and sync use case (+ provide logging and rest template)
• add fk&pk&unique constraints
• add schema for mysql and pgsql
• edit script: handle rename and path change, currently it will create a new script :D
• add samples scripts (e.g. making a http call to a rest end point for fetching the invoice number)
• add and wire up the following events: RESERVATION_CANCELLED, RESERVATION_EXPIRED: see who call ticketReservationRepository.remove
• add EVENT_CREATED hook point :)
• display error when trying to save invalid scripts
• refine the admin ui
• log errors somewhere -> check log table for plugins: -> show log in ui, add possibility to clean up?
• add parameters like the plugin system (see plugin_configuration table + MailChimpPlugin.getConfigOptions
• add mailchimp js script, add migration script
• remove all the previous plugin infrastructure

[syjer]

sample scripts

function getScriptMetadata() {
  return {async:true, events: ['RESERVATION_CONFIRMATION']};
}

function executeScript(scriptEvent) {
    log.warn('hello from script with event: ' + scriptEvent);
}

function getScriptMetadata() {
    return {async:false, events: ['INVOICE_GENERATION']};
}

function executeScript(scriptEvent) {
    return {invoiceNumber: '42'};
}

function getScriptMetadata() {
    return {async:false, events: ['INVOICE_GENERATION']};
}

function executeScript(scriptEvent) {
    var symbol = restTemplate.getForObject('https://api.coinmarketcap.com/v1/ticker/bitcoin/', Java.type('java.util.ArrayList').class)[0].symbol;
    return {invoiceNumber: symbol};
}

[syjer]

can be finally considered done...