Draft: Prevent auto-delete of epic children

What does this MR do?

When deleting an epic, the children automatically get destroyed as well due to ON DELETE CASCADE in the foreign key. This MR changes that value to SET NULL. See https://gitlab.slack.com/archives/C72HPNV97/p1614717313122300 (internal) for context.

It also changes the UX warning so the user is clear about the consequences of deleting the epic.

!55574 (merged) should be merged first as it's the "smallest first iteration"

related to #321752 (closed)

Screenshots (strongly suggested)

before

after

Database migration outputs

up

$ bundle exec rake db:migrate:up VERSION=20210302222539
== 20210302222539 ChangeEpicParentForeignKeyCascadeSetNull: migrating =========
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:epics)
   -> 0.0022s
-- execute("ALTER TABLE epics\nADD CONSTRAINT epics_parent_fk_nullify_on_destroy\nFOREIGN KEY (parent_id)\nREFERENCES epics (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0129s
-- execute("SET statement_timeout TO 0")
   -> 0.0014s
-- execute("ALTER TABLE epics VALIDATE CONSTRAINT epics_parent_fk_nullify_on_destroy;")
   -> 0.0065s
-- execute("RESET ALL")
   -> 0.0004s
-- remove_foreign_key(:epics, {:column=>:parent_id})
   -> 0.0039s
== 20210302222539 ChangeEpicParentForeignKeyCascadeSetNull: migrated (0.0364s) 

down

$ bundle exec rake db:migrate:down VERSION=20210302222539
== 20210302222539 ChangeEpicParentForeignKeyCascadeSetNull: reverting =========
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:epics)
   -> 0.0022s
-- execute("ALTER TABLE epics\nADD CONSTRAINT fk_25b99c1be3\nFOREIGN KEY (parent_id)\nREFERENCES epics (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0010s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE epics VALIDATE CONSTRAINT fk_25b99c1be3;")
   -> 0.0025s
-- execute("RESET ALL")
   -> 0.0004s
-- remove_foreign_key(:epics, {:column=>:parent_id})
   -> 0.0033s
== 20210302222539 ChangeEpicParentForeignKeyCascadeSetNull: reverted (0.0144s) 

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