Custom picker: Replace numbers with ticks, and display total number of selected pictures in title

[nicolas-raoul]

When you select 3 pictures in the custom picker, each selected picture has 3 in the corner.

As seen in #5221 this is confusing.

A better UI could be to display just a ✅ in the corner of each picture, and display something like Camera (3 selected) in the title (which currently only displays Camera or similar).

Having the number of selected pictures is quite important, as upload tends to fail when uploading more than N pictures (with N presumably depending on the environment).