[stable28] Also sort by getlastmodified when searching for photos

[backportbot-nextcloud]

Backport of #2226

[artonge]

/compile /

[smartbyte]

Sorry to re-open this case, but the sorting is still not correct in version 28.0.2 RC1 :-(
It now looks like the system is

1. picking files that have an exif date,
2. do an "order by"
3. adding that to the empty list
4. picking files with a "last-modified" date
5. do an "order by"
6. attaching it to the list.

What I would have needed:

1. picking files that have an exif date
2. picking files that have no exif date and use the last modified date
3. "order by" the "merged" date
4. and then attach it to the list

Would that be possible too?