Merge branch 'h/3.0.2'

[FIX] use jpg for screenshots

Merge branch 'r/3.0.1'

[NEW] Add mockups in readme

Merge branch 'r/3.0.0'

[NEW] update demo.gif

Merge branch 'f/update_dash' into develop

[FIX] Problem with colors in page comparison

[TST] add upload elements to tests

[REF] Update all pages to multi-output

[FIX] problem that deleted the global_df when data was not uploaded

[REF] reimplement update data

[REF] update show success message to new dash version

[TMP] try to unify callbacks

[REF] merge show_collapsibles in page upload