Display Vitrage Alarm Counts, by severity, in the Top Navbar of Horizon

Summary
=======
Display Vitrage Alarm Counts, by severity, in the Top Navbar of Horizon.

Motivation
==========
This improves UX as it makes important fault monitoring data easily visible
at all times when using Horizon (regardless of what Horizon panel/screen
the user is on) and provides a quick shortcut to the Vitrage active alarm
list for the associated detailed fault information.

Description
===========
When the Vitrage Dashboard is configured, Horizon will display the Critical,
Major, Minor and Warning active alarm counts from Vitrage in the Top
Navigation Bar of Horizon.
e.g.
   <project-menu> C:1 M:0 m:1 w:3 <settings/logout-menu>

- Being in the Horizon Top Navbar, the Vitrage Alarm Counts will be visible
  at ALL times, regardless of what specific Horizon panel/screen is being
  shown.
- Selecting the Vitrage Alarm Banner in the Top Navbar will take you directly
  to the Vitrage->Alarms panel showing the detailed list of Vitrage Active
  Alarms.

Vitrage supports multi-tenancy, i.e. some alarms are tenant-specific.
- the admin tenant will see counts related to ALL alarms,
- specific tenants will see counts related only to their alarms.

UX
==
The alarm counts banner
   - will be a single line,
   - will be located in the center of the Top Navbar,
   - will be roughly formatted as follows: C:1 M:0 m:1 w:3
     where C = Critical Active Alarm Count
           M = Major Active Alarm Count
           m = Minor Active Alarm Count
           w = warning Active Alarm Count
   - will have some color-coding, details TBD
        e.g. grey if counts are 0, red for critical alarm count > 0, etc.

Selecting the alarm counts banner will take the user directly to the
Vitrage->Alarms panel showing the detailed list of Vitrage Active Alarms.

Testing
=======
Ensure Alarm Counts Banner is not displayed if Vitrage Dashboard is not
configured.

Ensure Alarm Counts Banner is correctly updated if a new Vitrage Active Alarm
is raised or an existing Vitrage Active Alarm is cleared.

Ensure Alarm Counts Banner behaves correctly for both admin and tenant-specific
scenarios.

Outside Dependencies
====================
This blueprint has a dependency on the Vitrage blueprint, "Vitrage Alarm Counts
API", https://blueprints.launchpad.net/vitrage/+spec/alarm-counts-api ,
which provides a new Vitrage REST API to retrieve alarm counts for each
severity.

    /v1/alarm_counts/

    Method: Get

    Request Body
        all_tenants - (boolean, optional) shows the alarm_counts summed across
                      all tenants (in case the user has the permissions).

    Response Body
        Returns a JSON object containing the alarm counts for the different alarm
        severities.

    Response Example
        {
          "critical_alarm_count": 1,
          "major_alarm_count": 0,
          "minor_alarm_count": 1,
          "warning_alarm_count": 3
        }

Requirements Update Required
============================
None.

Doc Impact
==========
Horizon documentation related to the contents of the Top Navbar will be updated.