FIX: errors that're triggering by too long excerpts (#13056)

The excerpt field in the database is constrained to 1000 chars in length. To support this constraint we added a restriction that the topic_excerpt_maxlength setting must be between 0 and 999.

Unfortunately, sometimes it doesn’t work because:

- topic_excerpt_maxlength restricts the length of a visible to user excerpt. But we HTML-escape text before saving. If an excerpt contains & it’ll be &. One character for the user but 5 characters to save to the database. So if topic_excerpt_maxlength is set to 999 it’s not so hard to have an excerpt of for example 1003 characters in length and run into this issue.
- It’s possible to define a custom excerpt for a topic. Such excerpts bypass check for a length. So if the user defines a too long custom excerpt he will run into this issue.

Removing the constraint on the database level solves the problem. But we still need the constraint for topic_excerpt_maxlength on the setting page, because too long excerpts would make UI wonky.

db/post_migrate/20210513125608_remove_length_constrain_from_topic_excerpt.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class RemoveLengthConstrainFromTopicExcerpt < ActiveRecord::Migration[6.1]
+  def up
+    change_column :topics, :excerpt, :string, null: true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end