Search for tags should be case insenstive

Searches for tags on the [advanced search](https://vassalengine.org/library/search) page should be _case insenstive_.  There's no difference between searching for 

- `era:medival` and `era:Medival`
- `scale:tactical` and `scale:Tactical` 
- `theatre:france` and `theatre:France` 

Yours,
_Christian_
