Field mapping management

Hello,

Again, thanks a lot for all work done in Graylog, this product is awesome.

I'm linking here what i think should be in next milestone: https://graylog.ideas.aha.io/ideas/GL2E-I-453

As we assume, graylog drive it's own ES indices and already does some magic on it, i think, graylog should be able to offer to administrator the possibility to define fields conventions via dynamic template feature.

Concrete example, i define that every field ending by *_size is a number on ES.
- Graylog set this configuration in ES
- Graylog inform type conventions in /system/fields and so, somewhere in webui
- Graylog check this convention in a filter before pushing it in ES
- If validation fails, return to matching streams the "dead letters"

I think this feature can stay optional so does not break anything, but will helps a lot users to know in advance how to forge messages, and so get Numeric based widgets working. And also avoid administrators to manually set theses settings on indices "managed" by graylog.