Refactor: Extract two string utility methods

BlocksComponent::parseString() -> StringConverter::parseString()
BlocksComponent::stringToArray() -> StringConverter::stringToArray()

Closes #320
Added a setting for asset timestamp - closes #287

allow to manage this setting from the admin area with
default value to "force"
Fix error during fresh installation

With fresh installation, we cannot instantiate Settings.Setting,
as it will automatically attach some behaviors, eg: Trackable that
assumes that Users plugin is already installed.

Workaround: Use AppModel to save data
Log migration errors
Merge branch 'pr-327' into 1.6
Add record filtering and `DataMigration::loadFile()`

Useful to load a single record from a specific data file
Move new setting data install Install/Config/Data/SettingData.php

- Works for fresh install and incremental migration
- Centralize new data
- Addresses issues in https://github.com/croogo/croogo/commit/36ebb5cfb6844b23205347392acb515ae206a828#commitcomment-5199626

Closes #287
Fix: Incorrect constant symbol
Install: Abort if setupDatabase() failed
Fix: Mysql travis database setup
Fix: When deleting attachment, remove corresponding thumbnails

Closes #295
Merge branch '1.6-blocks' into 1.6
New class: CroogoStatus and PublishableBehavior

Closes #275
Adding Migration files to support Publishable for Blocks, Menus, and Nodes
Adding Publishable support for Nodes.Node
Adding Publishable support for Menus.Menu and Menus.Link
Adding Publishable support for Blocks.Block
Add date filtering feature to PublishableBehavior
Merge branch '1.6-preview-status' into 1.6
Implement MenusController::admin_toggle