Allow migrating old diagrams to the new inline and embeded

[oanalavinia]

Is your feature request related to a problem? Please describe or add steps to reproduce your use case.

After the implementation of #345 and #420 , the old diagram ways will be deprecated. Would be nice to have a way to migrate the old diagrams to this new format.

Most probably it could be divided in 2 separate issues: one for migrating diagrams and one for migrating diagram macros.

Describe the solution you'd like
I am not sure how this can be done, or if there are any technical constraints (e.g. on which page to migrate as inline?), but it would be nice to be able to migrate all at once, or at least in batches, not only one diagram at a time. To be checked after investigation

[Farcasut]

Could you specify some cases where you would like to migrate a standalone diagram to an inline one, and what you expect the result to be? Right now the diagrams are stored as standalone pages, would you expect to migrate them to inline diagrams on the same standalone page and move the content from the page content to an attachment? Or do you want to migrate only the ones that were created strictly via the diagram macro, in order to remove the hanging pages?

[ndascalita]

@Farcasut I would expect to be able to migrate the existing diagram macro into the diagram_inline one in order to remove the hanging pages