This patch does the following:
- Create new blocktype 'placeholder'
- Make sure it is copyable
- Make sure it is versionable
- Allow tags for it and that it works with create page via tags
- Populate instance config with content types using 'showmore' pagination

behatnotneeded

Change-Id: I8437fbab587b5dc8661512ce8d227161f0129475
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>