Drop unique requirement for template_id column in letter_versions table

[JamesMGreene]

Ran into some unexpected migration failures when running #147 against the production database. 😖

[JamesMGreene]

P.S. Props to @nawazkhan for suggesting I remove this in #147. I should've checked the production data more carefully. 😓

[JamesMGreene]

For posterity, this was the associated deployment failure (slightly scrubbed):

       migration file "20211230165827_harden-letter_versions-table.js" failed
       migration failed with error: alter table "letter_versions" add constraint "letter_versions_template_id_unique" unique ("template_id") - could not create unique index "letter_versions_template_id_unique"
alter table "letter_versions" add constraint "letter_versions_template_id_unique" unique ("template_id") - could not create unique index "letter_versions_template_id_unique"
Key (template_id)=(tmpl_idHasBeenScrubbedHere) is duplicated.