fix: template: enforce bounds of template max_ttl (#3662)

This PR makes the following changes:

- enforces lower and upper limits on template `max_ttl_ms`
- adds a migration to enforce 7-day cap on `max_ttl`
- allows setting template `max_ttl` to 0
- updates template edit CLI help to be clearer

coderd/database/migrations/000038_template_max_ttl_cap_7_days.down.sql

@@ -0,0 +1 @@
+-- this is a no-op

coderd/database/migrations/000038_template_max_ttl_cap_7_days.up.sql

@@ -0,0 +1,2 @@
+-- Set a cap of 7 days on template max_ttl
+UPDATE templates SET max_ttl = 604800000000000 WHERE max_ttl > 604800000000000;