Added support for nested repeaters in block

[yanhao-li]

This PR is to add the nested repeaters as discussed in #74

I found out the fields passed to afterSaveHandleBlocks have included all information we needed to support nested repeaters, it's merely not be handled properly.

This PR refactored the HandleBlocks trait a little bit by adding the following two methods:

• createBlock to create a block and recursively traverse it's child blocks and create them.
• getChildBlocks to recursively convert a child block's fields to the structure needed for creating a child block.

[ifox]

Merging as this adds a new feature without breaking changes but noting that BlockController@preview and HandleBlocks@hydrateHandleBlocks will also need to be updated for blocks and pages to render nested repeaters content during previews.