Normalised Field List

I believe that Graylog should have a list of the 'Normalised' fields within it. Implementing something similar the the grok pattern page, where a user has a standard set of normalised fields and can add new ones if required. When parsing a drop down would appear when a field name is required, so for example when a user types "%{WORD:" then a drop down would appear with all the field names in. If the field name wasn't already in the drop down then there should be an 'Add field' button or you could just add it in the 'Fields' page.

## Current Behavior
Currently when parsing, I have to refer back to an excel spreadsheet with all of my field names in.

## Context
By having a list of fields built into Graylog (that can be added to and transferred to other Graylog instances with contents packs) it would save me a lot of time when parsing as I have to refer to an excel sheet. It would also help eliminate human error as I have miss typed field names before.