## Summary

Detection Engine Meta Issue: elastic#50405

This PR adds the `Signals Histogram` component for use on the main `Detection Engine` page, `Rule Details` page, and the newly designed `Overview` page.

Out of the box configuration includes an `EuiSelect` for stacking by the following:
* Risk Scores
* Severities
* Event Actions
* Event Categories
* Host Names
* Rule Types
* Rules
* Users
* Destination IPs
* Source IPs

Additional configuration properties are available to configure the component as needed depending on where it will be displayed (e.g. no `Stack By` option on `Overview`, filter to specific `rule_id` on `Rule Details`, etc):

``` ts
interface SignalsHistogramPanelProps {
  defaultStackByOption?: SignalsHistogramOption;
  filters?: esFilters.Filter[];
  from: number;
  query?: Query;
  legendPosition?: 'left' | 'right' | 'bottom' | 'top';
  loadingInitial?: boolean;
  showLinkToSignals?: boolean;
  showTotalSignalsCount?: boolean;
  stackByOptions?: SignalsHistogramOption[];
  title?: string;
  to: number;
  updateDateRange: (min: number, max: number) => void;
}
```
##### Light Theme:
![de_hist_light](https://user-images.githubusercontent.com/2946766/71299977-41685800-234e-11ea-93bd-05a0c4cb6ee1.gif)

##### Dark Theme:
![de_histogram_dark](https://user-images.githubusercontent.com/2946766/71299980-45947580-234e-11ea-9d26-380bae5c4aa6.gif)


##### Overview:

Example props for overview impl:

``` jsx
<SignalsHistogramPanel
  filters={filters}
  from={from}
  loadingInitial={loading}
  query={query}
  showTotalSignalsCount={true}
  showLinkToSignals={true}
  defaultStackByOption={{
    text: 'Signals count by MITRE ATT&CK category',
    value: 'signal.rule.threats',
  }}
  legendPosition={'right'}
  to={to}
  title="Signals count by MITRE ATT&CK category"
  updateDateRange={updateDateRangeCallback}
/>
```
![image](https://user-images.githubusercontent.com/2946766/72030438-2fd7e900-3246-11ea-8404-40905ca5f85c.png)


Note @andrew-goldstein @angorayc @MichaelMarcialis -- looks like the MITRE ATT&CK Tactics are stored as a nested object in `signal.rule.threat`, so we may have to do some finangling to get it to show on the histogram. 

e.g. format:

``` json
{
  "framework": "MITRE ATT&CK",
  "tactic": {
    "id": "TA0010",
    "reference": "https://attack.mitre.org/tactics/TA0010",
    "name": "Exfiltration"
  },
  "techniques": [
    {
      "id": "T1002",
      "name": "Data Compressed",
      "reference": "https://attack.mitre.org/techniques/T1002"
    }
  ]
}
```




### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)
- [x] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)
- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials
  * Will work with @benskelker on any specific documentation
- [ ] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios
- [ ] ~This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~

### For maintainers

- [ ] ~This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~
- [ ] ~This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~