* Change `age:` metatag to require time units. This means e.g.
`age:<600` no longer works; instead you have to say `age:<600sec`.
* Allow time units in the `age:` metatag to be abbreviated as long as
they're unambiguous. This means `age:<60sec`, `age:<5min`, and
`age:<5mon` now work, in addition to `age:<60s` and `age:<60seconds`.
* Allow the `ratio:` metatag to be written like `ratio:16/9` in addition
to `ratio:16:9`.
* Fix invalid date searches like `date:foo` or `date:05-15-2021`
to return nothing instead of raising an "undefined method
'beginning_of_day' for nil" exception. (`date:05-15-2021` is invalid
because it's parsed as DD-MM-YYYY).
* Fix invalid searches like `score:foo`, `ratio:foo`, and `mpixels:foo`
to return nothing instead of being treated like `score:0`, `ratio:0`,
`mpixels:0`.
* Fix `age:<60m` to return nothing instead of silently being treated
like `age:<60seconds`.
* Fix `age:foo` to return nothing instead of silently being treated like
`age:0d` (return all uploads from today).
Fixes#4389.
