Implement Multiprocesses components

[selankon]

It implements necessary components to render multiple elections on the same page.

It depends on vocdoni/ui-components#204

If the metadata of chained elections contains the RenderWith information, it get the elections to render together.

Some considerations

• Won't work with anonymous voting
• Vote overwrite not tested

But should work out of the box with all other chained process and Vocdoni features.

It also implements a parameter to add a custom form validation function, to test anything needed between the choices of the elections.

[elboletaire]

Shouldn't be required to do this, since it's already declared in ui-components (use that instead).

[selankon]

02c95a3

[elboletaire]

Set as draft, since the dependency is a draft too