DM-11856: framework for client properties defined at build time

[loitly]

https://jira.lsstcorp.org/browse/DM-11856

• define '__$' as a prefix to indicate client property
• added getProp() to WebUtil.js to access the property on the client
  See help.base.url in config/app.config as an example.

Everything should work as before. More changes under the sam branch in ife and suit as well.

[tgoldina]

Works good now.

May be we should look into circular-dependency-plugin for webpack. The circular dependency bugs are hard to notice, especially when the modules are loaded in different order in a test...

[robyww]

Look good.