NXD Show "about" footer on every page

We show in small font size the same info that is shown on sign_in page:

    "GitLab Nexedi Edition", "About GitLab" and "About Nexedi"

This is good to have and hereby-introduced about-footer area will be
also used in the next patch for ICP too.

app/assets/stylesheets/framework/mobile.scss

@@ -99,6 +99,7 @@
   }
 }
 
+// TODO adjust .page-with-sidebar .about-footer
 @media (max-width: $screen-sm-max) {
   .issues-filters {
     .milestone-filter, .labels-filter {

app/assets/stylesheets/framework/sidebar.scss

@@ -34,6 +34,16 @@
   }
 }
 
+.about-footer {
+  text-align: center;
+  padding: 10px;
+  a {
+    margin-left:  3px;
+    margin-right: 3px;
+  }
+  font-size: small
+}
+
 .sidebar-wrapper {
   .header-logo {
     border-bottom: 1px solid transparent;

app/views/layouts/_page.html.haml

@@ -28,3 +28,9 @@
       .content
         .clearfix
           = yield
+    .about-footer
+      = link_to "GitLab Nexedi Edition", "https://lab.nexedi.com/nexedi/slapos/blob/master/software/gitlab/software.cfg"
+      |
+      = link_to "About GitLab", "https://about.gitlab.com/"
+      |
+      = link_to "About Nexedi", "http://nexedi.com/"