add env option to limit channel finder PV results on query

[tynanford]

New variable - REACT_APP_CF_MAX_RESULTS to give the option to limit the number of results.

If the variable doesn't exist or is left blank, we should just leave the ~size argument off of the CF query

API call example - /ChannelFinder/resources/channels?~name=*&~size=5

[tynanford]

#75