Search for URLs in messages doesn't return any results

This issue has been migrated from [#3024](https://github.com/matrix-org/synapse/issues/3024).

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### Description

Message text search doesn't work with URL. Even if you search for URL exactly as it was in a message, search will not return any results.

### Steps to reproduce

- Post some link in some room, e.g. https://www.youtube.com/watch?v=oHg5SJYRHA0
- Enter the same link in the search box: https://www.youtube.com/watch?v=oHg5SJYRHA0
- Press enter

The search results will not include the message with the link, which is unexpected.

### Version information

- **Homeserver**: self-hosted
- **Version**: matrix-synapse==0.26.0
- **Install method**: pip
- **Platform**: Linux, PostgreSQL 9.6