Refine styles on Jira Connect app page

- Fix issue with iframe not spanning whole height
- Make "Sign in" text fixed so it doesn't scroll with content
- Add font-size to important text to make it smaller and
consistent with GitLab in general

app/assets/stylesheets/page_bundles/jira_connect.scss

@@ -45,14 +45,16 @@ $header-height: 40px;
 }
 
 .jira-connect-user {
-  float: right;
-  position: relative;
-  top: -30px;
+  font-size: $gl-font-size;
+  position: fixed;
+  top: 10px;
+  right: 20px;
 }
 
 .jira-connect-app {
   margin-top: $header-height;
   max-width: 600px;
+  min-height: 95vh;
   padding-top: 48px;
   padding-left: 16px;
   padding-right: 16px;
@@ -113,5 +115,6 @@ svg {
 }
 
 .browser-limitations-notice {
+  font-size: $gl-font-size;
   margin-top: 32px;
 }