PageKite: Get rid of tabs in the configuration page

All most all apps in FreedomBox now follow a flat page structure instead of tabs (with exceptions). Pagekite still uses tabs and is somewhat consistent. Remove tabs in the configuration page (visit /plinth/apps/pagekite after installation) and show all the configuration options in a single view with multiple sections. See GitWeb configuration page for inspiration on how a single page configuration view looks like.

• Remove the tabs by removing subsubmenu in views.py. This will show only the configuration form.
• Drop the Javascript code that expands and collapses the configuration when enable/disable checkbox is clicked. This inconsistent with other configuration pages we have.
• Drop the standards service configuration form. Remove any code that is related to configuring standard services. Ensure that all the standard services are always enabled. After pagekite is installed, after it is enabled/disabled, etc.
• Place the configuration form under the section title 'Configuration'. See Gitweb app for how to style this.
• Add a section with title 'Custom Services' in the main page. This should be below the 'Configuration' section. Show the list of custom services below it. This is essentially moving the listing part from older 'Custom Services' tab to the front page.
• Move the add custom services form to a separate view. This view should look very simple and not have app description or tabs. See Gitweb's add repository form view as an example.
• Add a button 'Add Custom Service' under that 'Custom Services' section immediately after the title. This should redirect to the add custom services form view.
• Cleanup any code that is unused after the changes.

This task is nominated for 'Contributor Invite' (ping @dannyhaidar-guest):

• Task Area: Coding
• Effort Estimate: 4/5
• Skills Required: Python, Django views/forms/templates/urls