We add a new config setting to allow the admin to set a fixed
list of userIDs to track for all client side webapp metrics.
This gives the admin to get a deeper look at how the application
is behaving for a single user.
A new section in the system console is also added for the user
to edit this setting from the UI.
https://mattermost.atlassian.net/browse/MM-61888
```release-note
A new config setting MetricsSettings.ClientSideUserIds is added
where you can set the user ids you want to track for client side webapp
metrics.
```
* fix lint errors
```release-note
NONE
```
* fixing tests
```release-note
NONE
```
1a58f923e0