Search is case-sensitive in non-English languages

This issue has been migrated from [#3116](https://github.com/matrix-org/synapse/issues/3116).

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### Description

It's hard to find the text you need in languages other than English as case becomes important.

### Steps to reproduce

- in a non-English room try to search for a word in lower and upper case
- the results would be quite different
- searching for an English word in upper and lower case shows the same result

Also, [here](https://github.com/matrix-org/synapse/blob/eb53439c4a4fbfc8714a187f3491a3bac584eae5/synapse/storage/search.py#L320) the FTS language is hardcoded as English so no stemming is supported for other languages. I propose lowercasing the event text before inserting it and do the same on querying.

### Version information

- **Homeserver**: own homeserver
- **Version**: 0.27.4
- **Install method**: package manager
- **Platform**: Debian 9.4 on a VDS