Fix app size to make scrollbars visible

Fixes footer at the bottom of the page for all pages and changes
scrolling behavior for diff page.

On the diff page:
- horizontal scrolling is right above footer when required
- all headers stay in place when scrolling

Bug: Issue 4491
Change-Id: I62fdf130376b210c26fadcadc90bf452361ac15d

polygerrit-ui/app/elements/gr-app.html

@@ -41,8 +41,8 @@ limitations under the License.
     <style>
       :host {
         display: flex;
-        min-height: 100vh;
         flex-direction: column;
+        height: 100vh;
       }
       gr-main-header,
       footer {
@@ -58,6 +58,7 @@ limitations under the License.
       main {
         flex: 1;
         position: relative;
+        overflow: scroll;
       }
       .errorView {
         align-items: center;