Do not index already indexed files

[skorasaurus]

Relatedly,

Do not index already index files... There are some cases where this would be beneficial (e.g. if/when upgrading the whoosh

[skorasaurus]

https://github.com/opencleveland/drocer-webapp/blob/9c71aabe74a0c157205b7d4383a8205431d00829/tools/indexer.py