Fix/service catalog update schema mismatch

[Mary-Clb]

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes Field is_displayed_onservicecatalog not created on plugin update #210
• After the release of the Service Catalog display feature, some clients reported a fatal error on /ServiceCatalog during or just after plugin update.
  The root cause is a temporary schema mismatch: the code tries to use is_displayed_onservicecatalog before this column exists in glpi_plugin_news_alerts. Now, instead of a fatal SQL error, the plugin simply does not render alerts on the Service Catalog page until the schema migration is applied.

[Rom1-B]

You're only addressing the symptom, but clearly the root cause lies in the migration process, since the field isn't always added to the database.