Filter on last user modified (#8093)

Example: `userdate:PT1H` to select only articles modified by user during the last hour
Fix https://github.com/FreshRSS/FreshRSS/issues/4280#issuecomment-3393078024

Useful for instance to bulk mark as unread recently marked articles by error:
1. Click on the toggle button to show the read articles (making sure the toggle for the unread articles is off)
2. Sort by *User modified 9→1*
3. Filter by *user modified date*, for instance to the last 3 hours by typing `userdate:PT3H`
4. Click in the drop-down menu *Mark selection as unread*

P.S.: I have added at the same time a bunch of unit tests for date-related logic

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