Disable automatic reloading of assets in production

[robertknight]

In production builds, avoid stat-ing the manifest and bundle files on
each call to Environment.url since the assets never change until the
service is re-deployed.

See #4128 (comment)

FWIW I did some basic benchmarking of os.path.getmtime() locally. When all of the calls hit the stat cache, my system can manage ~1M calls/sec (native) or ~300K calls/sec (Docker container). On that basis I think it is quite unlikely that this would ever end up being a bottleneck, but adding a flag doesn't add much complexity either.

[codecov-io]

Current coverage is 83.45% (diff: 88.88%)

Merging #4138 into master will increase coverage by 0.02%

@@             master      #4138   diff @@
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  Files           158        158          
  Lines          6183       6197    +14   
  Methods           0          0          
  Messages          0          0          
  Branches        701        705     +4   
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+ Hits           5159       5172    +13   
+ Misses          949        948     -1   
- Partials         75         77     +2   

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