Emails: use https for links and images (#25957)

* emails: use an https image for the logo

Some mail clients do not like http images.

* emails: use an https:// link for "getting started"

* email: doc.grafana.com for Getting Started link

review suggestions from https://github.com/grafana/grafana/pull/25957

Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

emails/templates/layouts/default.html

@@ -107,7 +107,7 @@ td[class="stack-column-center"] {
 						            <table class="twelve columns">
 						              <tr>
 						                <td class="twelve sub-columns center">
-                              <img class="logo" src="http://grafana.org/assets/img/logo_new_transparent_200x48.png" style="width: 200px; float: none; display: inline">
+                              <img class="logo" src="https://grafana.com/assets/img/logo_new_transparent_200x48.png" style="width: 200px; float: none; display: inline">
                             </td>
                             <td class="expander"></td>
                           </tr>

emails/templates/welcome_on_signup.html

@@ -29,7 +29,7 @@
 				<tr>
 					<td class="center">
 						<p>
-							If you are new to Grafana please read the <a href="http://docs.grafana.org/guides/gettingstarted/">Getting Started</a> guide.
+							If you are new to Grafana please read the <a href="https://docs.grafana.com/guides/gettingstarted/">Getting Started</a> guide.
 						</p>
 					</td>
 					<td class="expander"></td>
@@ -46,4 +46,3 @@
 	</tr>
 </table>
 
-