Provide ability for plugin to store global configuration

[srinivasupadhya]

some plugins might need ability to store configuration. eg: github plugin might need username & password for updating PR status, artifact cleanup plugin might want to take configuration of "how many instances to keep" etc.

Currently this is being done through system property, configuration file (in location specified by plugin) etc. If Go server provides standardized approach to store configuration all plugins could take advantage of this.

---

Implementation:

• Plugin provides "configuration" keys & settings template
• Go Server on the plugins listing page provides a "configure" icon (super admins only), on click of which it renders pop-up with template & data (if its already stored)
• On save the data (key-value pairs) is passed to plugin for validation. On success they are stored as JSON to database
• Plugin will be able to access the configuration through GoApplicationAccessor message "get-configuration"

The data will be stored in a seperate table plugin which has id as plugin-id & column data which is JSON representation of key-value pairs

[mdaliejaz]

Verified. Works fine on 15.2.0(2093-c20772b).