diagnostics: Move diagnostics button to a dropdown menu

Currently a 'Diagnostics' button is shown in every app that supports it. The way it is shown it is one of first buttons, even before configuration of the app, that the user see. Yet, it is rarely used and is needed only when something wrong happens with the app. De-emphasize the prominence of the button to simplify app's appearance. Use the following design:

Design 1

Design 2

Tasks

• Define a new block called 'toolbar' in the app.html after manual page and before subsubmenu.
• Split the clients.html into two parts, button and the information table. Keep the information table in the clients.html and move the button into toolbar block.
• Move the diagnostics button from 'status' block to the toolbar block. Remove the diagnostics block.

This design is contradictory to #1472 (closed) . Note that for fine styling we should simply stick to forkawesome icons and bootstrap style instead of following designs here.