feat(webapp/core): generalize plugin api

Generalize cockpit plugin api into general "App" Plugin Api.

related to #CAM-1959
feat(webapp/core): server side Admin plugin api

provide Admin plugin facilitites:

- Plugin SPI is "AdminPlugin"
- Custom Command Executor is not provided ATM

related to #CAM-1960
feat(engine): ManagementService.setProperty()

- add possibility to set properties through the management service.

related to #CAM-1957
feat(webapp/plugin): conditional resource overrides

- Add PluginResourceOverride interface
- Make it possible for a plugin to provide list of
PluginResourceOverrides

related to #CAM-1958
feat(webapp/admin): client side plugin support

- Provide Admin Client Plugins Filter for injecting plugin JS assets
into admin app
- Load plugin REST Resources in AdminApplication
- Add new navigation entry "System". The new page is extensible through
plugins (Views)
- Client side extension point is "admin.system"

related to #CAM-1961
chore(webapp): Engine name in Http Request Headers

Set process engine name as HEADER 'X-Authorized-Engine' in all GET
requests made through angular $http.

This is required by the EE license check plugin.

related to #CAM-1882
chore(webapp/admin): add .monospaced CSS class

- add css class

related to #CAM-1882