Add in:users as a search filter to limit searches to users

[AndyScherzinger]

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Is your feature request related to a problem? Please describe.
Currently when you have large installation in terms of users and you need to search for a user, you can only do that via unified search. This works and filters down the list of users in case you are at */settings/users on your instance. The downside is that this then also trigger searches in all available search providers.

Describe the solution you'd like
Provide a app-filter in:users that can be added to the search query like it can be done for other apps like files and within the actual user search just remove any occurrence of in:users and just search with the rest of the search string (trimmed). Due to the in-filter all search providers will be skipped which is what is the desired behavior.

Describe alternatives you've considered
Implement an in-app search in users like present in talk but that would be way more work.

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This shall be backported down to v25

[sorbaugh]

Update. Original solution included a User search result list in the search dialog. However, expected behavior still unclear about what should happen when we click on a user (among other things). So we decided to reduce the scope to the original intent of this issue and just add the SearchProvider to have the in:users filter and revisit adding the search result list and how it should behave with a new issue.

EDIT: Will create new commit instead of ammend for future reference.