Annotation create API crashes on long URLs (OperationalError index row size exceeds maximum)

https://sentry.io/organizations/hypothesis/issues/956191704/?project=37293&referrer=github_plugin

```
OperationalError: (psycopg2.OperationalError) index row size 3216 exceeds maximum 2712 for index "uq__document_uri__claimant_normalized"
HINT: Values larger than 1/3 of a buffer page cannot be indexed.
Consider a function index of an MD5 hash of the value, or use full text indexing.
[SQL: 'INSERT INTO document_uri (created, updated, claimant, claimant_normalized, uri, uri_normalized, type, content_type, document_id) VALUES (%(created)s, %(updated)s, %(claimant)s, %(claimant_normalized)s, %(uri)s, %(uri_normalized)s, %(type)s, %(content_type)s, %(document_id)s) RETURNING document_uri.id'] [parameters: {'uri_normalized': u'httpx://pdf.sciencedirectassets.com/278653/1-s2.0-S1877705811X00164/1-s2.0-S1877705811029535/main.pdf?AWSAccessKeyId=ASIAQ3PHCVTYQUUTX7TQ&Expires=15 ... (1292 characters truncated) ... ZZYEEjAgiZEn3iJkF9RJF42+wzdHidDMB0zXFzcOFaRJ0+Em2wfBQkCGLh6j6WeLnBwoo5jFNLoKGLbb0JX1on4tX%2FMw%2FLSnJ2Cu38asyVXlAnviKGKaZXN+At0q04zZmLUQoNyYHEMQVQKQ=', 'updated': datetime.datetime(2019, 4, 13, 20, 5, 24, 263538), 'created'...
(29 additional frame(s) were not displayed)
...
File "sqlalchemy/util/compat.py", line 202, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb, cause=cause)
File "sqlalchemy/engine/base.py", line 1182, in _execute_context
context)
File "sqlalchemy/engine/default.py", line 469, in do_execute
cursor.execute(statement, parameters)
File "newrelic/hooks/database_psycopg2.py", line 51, in execute
**kwargs)
File "newrelic/hooks/database_dbapi2.py", line 25, in execute
*args, **kwargs)
```