TASK: Update topic URLs

[johnhenley]

Topic URLs are stored in table `activeforums_Topics'.

• Add persisted computed column URL_Hash using hashbytes and md5

• add index on URL_Hash

• All sql changes go into SqlDataProvider file with version and issue number in the file name

• In the TopicController add a FindByURL function to use the computed hash for speed and then non-hash URL for deduplication and accuracy, returning a matching topicinfo object