enh: Show feedback when uploading invalid file

[roshnet]

Description
If a user attempts to upload a file that is larger than the max file size, nothing happens. A static text exists, but it's better to have an interactive feedback for the user.

Steps to reproduce
Open the application portal and go to step 4 (documents upload). Try to upload a file > 500KB in any section.

Expected behavior
A snackbar, or another form of notification should popup for a few seconds to notify the user about the exceeding file size.
The same popup can also be shown when uploading a non-pdf file.