User search in MySQL store does not use full text search index

[e-chip]

Summary

Our team is using mattermost with mysql storage. And we found that we cannot find users by non-first word in the field.
For example if Firstname field is 'Maria Eleni' we cannot find her by 'Eleni' query.
SqlUserStore creates full text search index for all search modes.
But as i can see from the code user search in MySQL store does not use full text search index.

Steps to reproduce

Add user with two words in the firstname or lastname.
Try to find user (Ctrl+K or Ctrl+Shift+K) using the second word from the selected field.
User will not be found.

Expected behavior

User should be found.

Possible fixes

We have fixed this issue for us and if core team and community wants i can make a PR.
Also PR in mattermost-redux repo will be required to handle search results.

[e-chip]

Also i think these issues in Jira are related to this:
https://mattermost.atlassian.net/browse/MM-5466
https://mattermost.atlassian.net/browse/MM-8423

[amyblais]

@e-chip Thanks, I'm sure our team would be happy with a fix for this if you're interested to open a pull request.