Feature: Allow limiting the length of text that will go on the slide

[Károly Balogh]

When someone uploads an entry, limit the info text length, so the slide won't look like a wall of text that no one reads/too small to read. I.e. guide the user how much text is meaningful to add. Since every party might have their own preference, make this option configurable.

Not sure if the same should be added for entry title too, see. :)