Removes all "Link to additional documentation" on forms

[bertocq]

Background

With the new feature for upload documents (and after fixing the multiple-document upload bug) the "Link to additional documentation" input on forms should be removed.

What we need

• Remove external_url from proposals form (for users and management views)
• Remove external_url from budget investments form (for users and management views)
• Create a rake task to move all existing external_url to description of the item (see screenshot)

Screenshot

This is the old external url

After the rake task is moved to description

And the users use the new docs tab

Notes

Add a note at Deprecated section of Changelog to warn about those external_url columns being removed from database in the v0.14 release, and the necessity to run the rake task after migrating to v0.13

[raul-fuentes]

I'll do this one

[javierm]

Closed via #3397.