Searchabletext indexer fix

[petschki]

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remove SearchableText indexer and use IDynamicTextIndexExtender only. See #112 for discussion.

[petschki]

@mauritsvanrees I've updated mosaic to enable the behavior for the types. see plone/plone.app.mosaic#687

[mauritsvanrees]

Thanks, this works for me.

[mauritsvanrees]

Applying the plone.app.contenttypes profile is not needed anymore. Same for the other lines that load plone.app.contenttypes.
But that is unrelated to this issue. Just noting it.