Add "Yesterday" and "Today" as relative time spans for search

At present, there is no relative time span for "Yesterday" or "Today". These are a standard option in other log monitoring platforms.

## What?

For events, these would default to the root user timezone. For users, these would relate to the logging in user timezone.

Today should capture the present date, from 00:00 up to the present time.

Yesterday should captures yesterday's date, from 00:00 to 24:00.

## Why?

This is a standard feature of competitior platforms.

In my experience running a Splunk cluster for a buisness, these were the most-used relative time span for dashboards.

Without the addition of this feature, it is not possible for users to create a dashboard element that accurately measures Graylog Enterprise liscence usage, or triggers alerts off the same.

These are highly intuitive spans of time for human understandng of data.