Field chart default resolution differs from search result histogram

### Problem description

Since field charts are using the same time range parameters as the whole search, we also need to use the same default resolution as the search result histogram, instead of using `minute` when no resolution is selected.

In my opinion, we should also let the user override the default resolution for field graphs, so once the user selects a resolution from the field graph options, we should always use that one.
### Steps to reproduce the problem
1. Perform a search in the last 5 minutes
2. Create a field chart
3. Change the search time range to the 7 days
4. Observe how the resolution for both graphs differs
### Environment
- Graylog Version: 2.0.0-beta.2-SNAPSHOT