Add configurable global user timeout configuration setting

[danotorrey]

What?

Currently, the session timeout is configurable for each user on the Users > Edit page in the Sessions Timeout field.

A customer is requesting the ability to edit the timeout globally for all users at once.

Why?

A configurable global timeout would prevent the work required to edit and maintain the session timeout for individual users.

Ticket reference: HS-579202485

[danotorrey]

Perhaps we could consider adding a server.conf property to control the global timeout currently specified in a code constant here:

graylog2-server/graylog2-server/src/main/java/org/graylog2/users/UserImpl.java

Line 113 in e5b9889

public static final long DEFAULT_SESSION_TIMEOUT_MS = TimeUnit.HOURS.toMillis(8);

[patrickmann]

Instead of adding an entry to the server.conf file which needs to be kept in sync across all nodes, we will make this a new runtime configuration setting.