Date range filters with a relative date and unbound upper or lower limit

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**Is your feature request related to a problem? Please describe.**

Right now, the only direct way to define a date range filter with a date and an upper or lower bound is by using a fixed date:

Add an option for users to set up filters for unbounded time ranges with relative dates.

**Describe the solution you'd like**
Ideally, users should be able to specify:
- a relative starting point without needing a fixed end date: e.g., "starting X {units} ago", or "starting X {units} in the future"
- a relative end point without needing a fixed start date: e.g., "before X {units} ago", "before X {units} in the future"

**Describe alternatives you've considered**
In some cases it is possible to set up a relative range using a relative date and specifying a very early or late start date for the interval (see the additional context below), but this is unintuitive, and it is not generic enough to cover all possible ranges.

**How important is this feature to you?**
Requested by a Pro customer.

**Additional context**
From customer:
> I need to allow users to filter a dashboard by a relative range. Our use case is for dashboards tracking "staleness" - for example, accounts with no activities in the past x days.
>
> The best I've found is "previous 9999 days and x days ago" like below, but it looks ridiculous and unprofessional. Can we put something else besides 9999 in there so it's just "starting x days ago"? Or is there another way to do this?

No files, tests, or entry points are named in the issue. Start by mapping the dashboard date-filter behavior described here, then define done as supporting relative unbounded starts and ends without placeholder dates such as 9999 days, with coverage for past and future relative dates.

Written by the indexing model from the issue text.