PLT-6090 adding ability to read license file from disk (#5895)

* PLT-6090 adding ability to read license file from disk

* Fixing unit test that fails only sometimes

* Fixing test that fails randomly

.gitignore

@@ -5,6 +5,8 @@ node_modules
 /dist
 /webapp/dist
 npm-debug.log
+mattermost.mattermost-license
+config/mattermost.mattermost-license
 
 web/static/js/bundle*.js
 web/static/js/bundle*.js.map