Html2markdown db migration

Simple alembic script to carry out the migration from html to markdown for app long descriptions in old apps.

Refs #702, #684

Notice that both this and #707 are branching the alembic history, with must be linear, as they both share the same downgrade revision (or parent revision). The solution to this will be that right after any of the 2 is merged, it will turn to be the real parent of the other one (the one that have not been merged yet) and so we can keep a linear database migration history. EDIT: it has been modified here, as #707 has been merged to master.