Dashboard: Support configuring default timezone via config file (#27404)

Add a default timezone that the administrator can set in the settings. 
This setting is be used as default for the users timezone preference.
Can be used when creating Grafana instances without administrator 
intervention, in order to give user the correct default timezone.

Fixes #25654

docs/sources/administration/configuration.md

@@ -1426,3 +1426,6 @@ interval_year = YYYY
 Set this to `true` to have date formats be automatically be derived from browser locale. Defaults to `false`. This
 is an experimental feature right now with a few problems that remain unsolved.
 
+### default_timezone
+
+Used as the default timezone for user preferences. Can be either `browser` for the browser local timezone or a timezone name from IANA Time Zone database, e.g. `UTC` or `Europe/Amsterdam` etc.

docs/sources/administration/preferences.md

@@ -32,7 +32,7 @@ Your Grafana preferences include whether uses the dark or light theme, your home
 1. In the Preferences section, you can edit any of the following:
    - **UI Theme -** Click to set the **Dark** or **Light** to select a theme. **Default** is either the dark theme or the theme selected by your Grafana administrator.
    - **Home Dashboard -** Refer to [Set your personal home dashboard]({{< relref "change-home-dashboard.md#set-your-personal-home-dashboard" >}}) for more information.
-   - **Timezone -** Click to select an option in the **Timezone** list. Refer to [Time range controls]({{< relref "../dashboards/time-range-controls.md" >}}) for more information about Grafana time settings.
+   - **Timezone -** Click to select an option in the **Timezone** list. **Default** is either the browser local timezone or the timezone selected by your Grafana administrator. Refer to [Time range controls]({{< relref "../dashboards/time-range-controls.md" >}}) for more information about Grafana time settings.
 1. Click **Save**.
 
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