deposit: form page customization

* Increases customization in the form page by adding new blocks and spliting the form itself onto a new file.

* Tested both on Invenio Demosite and in Inspire instance.

* Closes #1988.

Probably with @greut's lastest PR #2004 I should rebase it again, so when he gets merged, ping me @jirikuncar so I can update it. ;)