Searching users is not intuitive, can't do partial searches without special characters

[C0rn3j]

Steps to reproduce

1. Have a user with display name "Martin Rys"
2. Go to https://cloud.example.com/settings/users as an admin and try to search for "Rys"
3. 0 results

Expected behaviour

"Martin Rys" should pop up because "Rys" is part of the string "Martin Rys"

Actual behaviour

No results. Doing "*Rys" gets the result but that is far from intuitive.

Server configuration

Operating system: Ubuntu 18.04

Web server: Nginx

Database: MariaDB

PHP version: 7.2.10

Nextcloud version: 15.0.2

Updated from an older Nextcloud/ownCloud or fresh install: Originally NC 12

Where did you install Nextcloud from:
Manually.

Client configuration

Browser: Firefox 65

Operating system: Arch Linux

Logs

N/A, will provide if they're actually needed for the search issue.

[putt1ck]

NB I think I'm going to remark this a bug as we can show that the results are inconsistent regarding the wildcard/partial match user searches. Scenario:

installation has a user (from LDAP) username john.smith, display name John Smith, the user is a member of several groups;
a search for "john" returns the user regardless of the group searched in;
a search for "smith"returns the user in some groups but not ones with many users (including the "everyone" category, installation has >400 users from LDAP, maybe 10 local to NC database);
a search for "*smith" returns the user from the large groups but not the small ones.

Further:

a search for "a" returns all LDAP users with first letter a - and all NC database users whose username, display name or email address contain "a".

[putt1ck]

NB that's NC16.04, updated in many steps from ~10. LDAP server is samba, backend DB is postgres.

[skjnldsv]

Duplicate of #11540