V1_9 build fix - add core-js

[hkz-aarvesen]

This fixes bug 540.

When building with the v1_9 branch, you will get the following error:

[INFO] ERROR in ./src/modules/helpdesk/main.ts
[INFO] Module not found: Error: Can't resolve 'core-js/es6/set' in '/app/client/src/modules/helpdesk'
[INFO]  @ ./src/modules/helpdesk/main.ts 7:0-26
[INFO] 
[INFO] ERROR in ./src/modules/peoplesearch/main.ts
[INFO] Module not found: Error: Can't resolve 'core-js/es6/set' in '/app/client/src/modules/peoplesearch'
[INFO]  @ ./src/modules/peoplesearch/main.ts 6:0-26
[ERROR] npm ERR! code ELIFECYCLE
[ERROR] npm ERR! errno 2
[ERROR] npm ERR! ng1-webpack@0.0.1 build: `webpack --mode=production`
[ERROR] npm ERR! Exit status 2
[ERROR] npm ERR! 
[ERROR] npm ERR! Failed at the ng1-webpack@0.0.1 build script.
[ERROR] npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

Adding core-js explicitly to the package.json results in a succesful build.

[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for PWM Password Self Service 2.0.0-SNAPSHOT:
[INFO] 
[INFO] PWM Password Self Service .......................... SUCCESS [02:28 min]
[INFO] PWM Password Self Service: Server JAR .............. SUCCESS [07:04 min]
[INFO] PWM Password Self Service: Angular Client JAR ...... SUCCESS [08:11 min]
[INFO] PWM Password Self Service: Server WAR .............. SUCCESS [05:16 min]
[INFO] PWM Password Self Service: Executable Server JAR ... SUCCESS [ 25.273 s]
[INFO] PWM Password Self Service: Data Service WAR ........ SUCCESS [ 36.937 s]
[INFO] PWM Password Self Service: REST Test Server WAR .... SUCCESS [ 10.558 s]
[INFO] PWM Password Self Service: Docker Image ............ SUCCESS [01:05 min]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  25:19 min
[INFO] Finished at: 2020-10-23T18:55:59Z
[INFO] ------------------------------------------------------------------------