Riot 0.5.3 #master 86 Searching in This Room/All Rooms yields no results

### Description
Riot Desktop 0.12.2 (macOS) Searching in This Room/All Rooms yields no results

### Steps to reproduce
- Enter any room
- Look for a string that's definitely been said in that room, or just say “testing foo” in that room.
- Now we know a string that should be found.
- Go to the search item by clicking on the loupe icon of the room
- in the search field enter “testing foo” (sand the quotes, as usually)
- klick either “This Room” or “All Rooms” to define the search scope
- wait for the results to be displayed

Describe how what happens differs from what you expected.
Nothing is ever found, or at least nothing is displayed. Even strings/words that *definitely* must be there, like “testing foo” are not found.

### Version information
For the desktop app:

- **OS**: macOS 10.12.6 (Sierra)
- **Version**: 0.12.2

- **OS**: iOS 10.3.3
- **Version**: 0.53 #master 86; Olm 2.2.2

- **Server**: synapse 0.22.1 matrix.org or other federated homeservers on the same version.

### Regression
This did indeed work with former versions of Riot Desktop and iOS and seems to be a regression in 0.12.x./0.5.3.
Related Bug in Riot/Desktop/macOS #[4998](https://github.com/vector-im/riot-web/issues/4998)

Start by reproducing room and all-room searches using the versions and macOS/iOS details in the report, then trace the iOS search entry point. The issue names no files or tests, so first identify the search implementation and existing regression coverage. Done means messages such as “testing foo” appear in both selected scopes, or the historical version limitation is documented.

Written by the indexing model from the issue text.