Bug 35153: Convert IntranetmainUserblock system preference to additional contents

Bug 35153: Convert IntranetmainUserblock system preference to additional contents

This patch moves the IntranetmainUserblock system preference into HTML
customizations, making it possible to have language- and
library-specific content.

To test you should have some content in the IntranetmainUserblock
system preference before applying the patch. Apply the patch and run
the database update process.

- In the staff client, go to Tools -> HTML customizations and verify
  that the content from IntranetmainUserblock is now stored there.
- The HTML customization entry form should offer IntranetmainUserblock
  as a choice under "Display location."
- Update and reinstall active translations (for instance fr-FR):
  - perl misc/translator/translate update fr-FR
  - perl misc/translator/translate install fr-FR
- Enable the translation if necessary under Administration -> System
  preferences -> language.
- Edit the IntranetmainUserblock HTML customization and add unique
  content to the "fr-FR" tab.

- Go to the staff client home page. You should see the
  content you added to the IntranetmainUserblock HTML customization.
- Switch to your updated translation and confirm that the content you
  added for your translation shows up correctly.
- Go to Administration -> System preferences and search for
  "IntranetmainUserblock." It should return no results.

Sponsored-By: Athens County Public Libraries
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>